Symptoms of infectious diseases

Diseases & Conditions A-Z Index

  • Abdominal Aortic Aneurysm — see Aortic Aneurysm
  • Acanthamoeba Infection
  • ACE (Adverse Childhood Experiences)
  • Acinetobacter Infection
  • Acquired Immune Deficiency Syndrome (AIDS) — see HIV
  • Acute Flaccid Myelitis (AFM)
  • Adenovirus Infection
  • Adenovirus Vaccination
  • ADHD
  • Adult Vaccinations
  • Adverse Childhood Experiences (ACE)
  • AFib, AF (Atrial fibrillation)
  • AFM
  • African Trypanosomiasis — see Sleeping Sickness
  • Agricultural Safety — see Farm Worker Injuries
  • AHF (Alkhurma hemorrhagic fever)
  • AIDS (Acquired Immune Deficiency Syndrome)
  • Alkhurma hemorrhagic fever (AHF)
  • ALS
  • Alzheimer’s Disease
  • Amebiasis, Intestinal
  • American Indian and Alaska Native Vaccination
  • American Trypanosomiasis — see Chagas Disease
  • Amphibians and Fish, Infections from — see Fish and Amphibians, Infections from
  • Amyotrophic Lateral Sclerosis — see ALS
  • Anaplasmosis, Human
  • Ancylostoma duodenale Infection, Necator americanus Infection — see Human Hookworm
  • Angiostrongylus Infection
  • Animal-Related Diseases
  • Anisakiasis — see Anisakis Infection
  • Anisakis Infection
  • Anthrax Vaccination
  • Anthrax
  • Antibiotic-resistant Infections – Listing
  • Antibiotic and Antimicrobial Resistance
  • Antibiotic Use, Appropriate
  • see also Get Smart about Antibiotics Week
  • Aortic Aneurysm
  • Aortic Dissection — see Aortic Aneurysm
  • Arenavirus Infections
  • Arthritis
  • Childhood Arthritis
  • Fibromyalgia
  • Gout
  • Osteoarthritis (OA)
  • Rheumatoid Arthritis (RA)
  • Ascariasis — see Ascaris Infection
  • Ascaris Infection
  • Aseptic Meningitis — see Viral Meningitis
  • Aspergillosis — see Aspergillus Infection
  • Aspergillus Infection
  • Asthma
  • Atrial fibrillation (AFib, AF)
  • Attention Deficit/Hyperactivity Disorder — see ADHD
  • Autism
  • see also Genetics and Genomics
  • Avian Influenza

Infections: What You Need to Know

Below, we’ll explore the different types of infections, list some examples you may be familiar with, and look into some possible treatments.

Viral infections

Viruses are very tiny infectious organisms. They’re even smaller than bacteria. On the most basic level, a virus is composed of a piece of genetic material that’s surrounded by a protein shell. Some viruses may have an additional envelope or other features on their surface.

Viruses are parasitic and require a host cell in which to carry out their life cycle. Once the virus has entered the host cell, it’s able to use cellular components to reproduce. New viruses are released from the host cell, a process that’ll sometimes cause the host cell to die.

Some examples of viral infections include:

  • influenza (the flu)
  • common cold
  • measles
  • rubella
  • chickenpox
  • norovirus
  • polio
  • infectious mononucleosis (mono)
  • herpes simplex virus (HSV)
  • human papillomavirus (HPV)
  • human immunodeficiency virus (HIV)
  • viral hepatitis, which can include hepatitis A, B, C, D, and E
  • viral meningitis
  • West Nile Virus
  • rabies
  • ebola

Possible treatments

Most of the time, the treatment of viral infections centers on relieving symptoms until your immune system clears the infection.

In some cases, antiviral drugs may be available to help treat a viral infection. Some examples of viral infections for which antivirals are available include HIV, herpes, and hepatitis C.

Some viruses stay with you for life once you’ve been infected. They can lie dormant within your body and may reactivate. Some examples include herpes simplex virus (HSV) and varicella-zoster virus (VZV).


It’s important to remember that antibiotics aren’t effective in treating a viral infection.

Bacterial infections

Bacteria are single-celled microorganisms. They’re very diverse, coming in a variety of different shapes and sizes.

Bacteria can be found in all sorts of environments, including soil, bodies of water, and in or on our bodies. Some can survive extreme temperatures or even radiation exposure.

Although there are a great many bacteria in and on our bodies, these bacteria often don’t cause disease. In fact, the bacteria in our digestive tract can help us digest our food.

However, sometimes bacteria can enter our bodies and cause an infection. Some examples of bacterial infections include:

  • strep throat
  • bacterial urinary tract infections (UTIs), often caused by coliform bacteria
  • bacterial food poisoning, often caused by E. coli, Salmonella, or Shigella
  • bacterial cellulitis, such as due to Staphylococcus aureus (MRSA)
  • bacterial vaginosis
  • gonorrhea
  • chlamydia
  • syphilis
  • Clostridium difficileC. diff
  • tuberculosis
  • whooping cough
  • pneumococcal pneumonia
  • bacterial meningitis
  • Lyme disease
  • cholera
  • botulism
  • tetanus
  • anthrax

Bacterial infections are most often treated with antibiotics. Antibiotics are medications that affect bacterial growth. They can either impede bacteria from multiplying or kill them outright.

There are different classes of antibiotics. The one you’re prescribed will depend on what type of bacterium is causing your infection. Additionally, misuse of antibiotics has caused many bacteria to develop resistance to them.

take as prescribed

If you’re prescribed antibiotics for a bacterial infection, take the entire course of antibiotics — even if you begin to feel better after a few days. Not doing this can prevent the infection from clearing and can contribute to antibiotic resistance.

Fungal infections

Fungi are another diverse group of organisms that can include things like yeasts and molds. They can be found throughout the environment, including in the soil, indoors in moist areas like bathrooms, and on or in our bodies.

Sometimes fungi are so small that you can’t see them with the naked eye. Other times, you’re able to see them, such as when you notice mold on your bathroom tile.

Not all fungi can make you ill, but some examples of fungal infections include:

  • vaginal yeast infections
  • ringworm
  • athlete’s foot
  • thrush
  • aspergillosis
  • histoplasmosis
  • Cryptococcus infection
  • fungal meningitis

Fungal infections can be treated with antifungal medications. The type of medication that you’re prescribed will depend on the type of fungal infection you have.

For example, a topical antifungal cream may be prescribed for conditions like ringworm or athlete’s foot. Oral antifungal medications are also available. More severe fungal infections may require intravenous (IV) antifungal medication.

Parasitic infections

Parasites live on or in a host organism and get food or other nutrients at the host’s expense. There are three types of parasites that can cause illness in humans:

  • Protozoa: small, one-celled organisms
  • Helminths: larger, worm-like organisms
  • Ectoparasites: organisms such as fleas, ticks, and lice

Some examples of infections that are caused by parasites include:

  • malaria
  • toxoplasmosis
  • trichomoniasis
  • giardiasis
  • tapeworm infection
  • roundworm infection
  • pubic and head lice
  • scabies
  • leishmaniasis
  • river blindness

As with bacterial and fungal infections, there are specific drugs available to treat a parasitic infection. The type of antiparasitic medication that you’ll need to take will depend on the type of parasite that’s causing your infection.


A prion actually isn’t an organism at all — it’s a protein. Prions can affect normal body proteins and cause them to fold into abnormal shapes. They can cause development of dementia and difficulties walking or speaking.

Prion diseases are very rare. Only 300 cases are reported in the United States each year.

While some prion conditions are inherited, others can be acquired through consuming contaminated food and are considered infectious. Examples include variant Creutzfeldt-Jakob disease (related to mad cow disease) and kuru.

There’s currently no curative treatment for prion diseases. However, there are medications that may slow their progression.


Major distinctions

The normal state of an organism represents a condition of delicate physiological balance, or homeostasis, in terms of chemical, physical, and functional processes, maintained by a complex of mechanisms that are not fully understood. In a fundamental sense, therefore, disease represents the consequences of a breakdown of the homeostatic control mechanisms. In some instances the affected mechanisms are clearly indicated, but in most cases a complex of mechanisms is disturbed, initially or sequentially, and precise definition of the pathogenesis of the ensuing disease is elusive. Death in humans and other mammals, for example, often results directly from heart or lung failure, but the preceding sequence of events may be highly complex, involving disturbances of other organ systems and derangement of other control mechanisms.

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The initial cause of the diseased state may lie within the individual organism itself, and the disease is then said to be idiopathic, innate, primary, or “essential.” It may result from a course of medical treatment, either as an unavoidable side effect or because the treatment itself was ill-advised; in either case the disease is classed as iatrogenic. Finally, the disease may be caused by some agent external to the organism, such as a chemical that is a toxic agent. In this case the disease is noncommunicable; that is, it affects only the individual organism exposed to it. The external agent may be itself a living organism capable of multiplying within the host and subsequently infecting other organisms; in this case the disease is said to be communicable.

Noncommunicable disease

Noncommunicable diseases generally are long-lasting and progress slowly, and thus they are sometimes also referred to as chronic diseases. They can arise from environmental exposures or from genetically determined abnormalities, which may be evident at birth or which may become apparent later in life. The World Health Organization (WHO) has identified four major types of noncommunicable disease: cancer, cardiovascular disease (e.g., heart attack, stroke), chronic respiratory disease (e.g., asthma), and diabetes mellitus. WHO estimates that, combined, these four groups of conditions account for 82 percent of all deaths from noncommunicable disease.

brain cancer; magnetic resonance imaging (MRI)An image, produced by magnetic resonance imaging (MRI), of a human brain affected by cancer. The bright blue area indicates that the cancer spread to the occipital lobe (lower right).© Photodisc/Thinkstock

Noncommunicable diseases that arise from inherited genetic abnormalities often leave an individual ill-equipped to survive without some form of treatment. Examples of inherited disease include cystic fibrosis, Down syndrome, and inborn errors of metabolism, which are present at birth. Examples of inherited diseases that emerge in adulthood include Huntington disease and certain forms of cancer (e.g., familial breast cancer involving inherited mutations in either of the genes BRCA1 or BRCA2).

According to the World Health Organization, as of 2011 there were 12,420 different diseases and health-related ailments. In other words, you should be thanking your immune system and modern medicine before you go to sleep each and every night.

Of those numerous diseases, perhaps none is more common than infectious diseases, which are defined by WHO as “any pathogenic microorganism that can be spread directly or indirectly from one person to another.”

In a given year, we are literally talking about billions of people that will contract an infectious disease. Keep in mind, of course, that this could be something as simple as the common cold, which is often nothing more than a nuisance, to something as serious as the HIV/AIDS virus, which has claimed nearly 14 million lives since the epidemic began in the 1980s. Based on WHO’s statistics, the combination of AIDS, malaria, and tuberculosis is responsible for half (approximately 5 million) of all infectious-disease deaths each year.

Image source: Getty Images.

With that in mind, today we’re going to look at the five most common infectious diseases around the globe. As we go through these infectious diseases, one by one, understand that much of this data is estimated because it’s difficult to get a bead on infection rates in developing countries where access to health-care can be limited. Therefore, the figures can be left up to a little interpretation based on the statistics from WHO and the Centers for Disease Control and Prevention.

In addition, we’ll also look at what, if any, global pharmaceutical companies may be able to take advantage of these multi-million-person, and even billion-person, opportunities.

So, without further ado, here are the five most common infectious diseases.

Hepatitis B
According to current statistics, hepatitis B is the most common infectious disease in the world, affecting some 2 billion people — that’s more than one-quarter of the world’s population. This disease, which is characterized by an inflammation of the liver that leads to jaundice, nausea, and fatigue, can lead to long-term complications such as cirrhosis of the liver or even liver cancer. The concern is primarily for those who carry the chronic form of the disease, which is estimated to be about 350 million people.

Perhaps one of the best known therapies here is Gilead Sciences’ (NASDAQ:GILD) Viread, which was approved in the U.S. in 2008 and blocks an enzyme that the hepatitis B virus needs to replicate in liver cells. According to Gilead’s third-quarter report, sales of the drug were up 11% through the first nine months over the previous year, and it should deliver more than $900 million in cumulative sales by the time fiscal 2013 is over. With a strong presence in AIDS therapies as well, Gilead is quickly becoming an infectious-disease juggernaut.

Another name to keep an eye on here is Dynavax Technoloogies (NASDAQ:DVAX). Although the FDA has been less than cooperative with Dynavax’s attempts to bring Heplisav to market, implying that the company may need to run an additional trial to satisfy its safety concerns, Heplisav had demonstrated impressive efficacy in clinical trials.

Malaria, a mosquito-borne disease that tends to affect children the most in tropical and subtropical climates, affects more than 500 million people annually and results in anywhere between 1 million and 3 million deaths. Behind hepatitis B, it appears to be the second most-common infectious disease, and it certainly is one of the most deadly on an annual basis.

Increasing public awareness of the dangers of mosquitoes in tropical and subtropical climates has helped somewhat, but malaria cases have unfortunately been on the rise again in recent years. The most common anti-malarial medication available is an oral therapy known as Lariam, which the U.S. Army invented in the late 1980s but was licensed to Roche (OTC:RHHBY), which sold it through August 2009. The drug is now sold in generic versions.

While a relatively effective first-line preventative treatment and second-line therapy following contraction of the disease, Lariam also has a laundry list of side effects, including serious neurological and psychiatric side effects that, in July, prompted the FDA to beef up its stance concerning Lariam with a black-box warning.

Hepatitis C
Hepatitis C is a less common and less severe form of hepatitis, but it almost always develops into a chronic, not acute, condition, unlike hepatitis B. Although only 3 million to 4 million new cases are reported each year, some 180 million people worldwide suffer from this chronic condition, which can lead to liver cancer or cirrhosis of the liver over time.

Advancements in the U.S. for treating hepatitis C have been nothing short of breathtaking over the past three years. Three years ago, the standard of treatment involved pegylated interferon and a ribavirin over the course of 24 or 48 weeks. The net result was a response (not a cure, just a response) in around 50% of HCV-positive patients. With the approval of Gilead Sciences’ Sovaldi earlier this month, patients with genotype 1 (the most common form of the disease) can expect a sustained virologic response, or SVR (an undetectable level of disease), after 12 weeks in more than 90% of cases.

In addition, AbbVie (NYSE:ABBV) is also developing its own direct-acting antiviral combo drug, which has demonstrated a 12-week SVR of greater than 90% as well in the tough-to-treat genotype 1 patient subset. It’s extremely rare to develop a cure for such a global disease nowadays, but we could well be on our way to one when it comes to treating hepatitis C.

Image Source: Getty Images.

It’s at times like these that we curse mosquitoes, because a very specific type of mosquito (Aedes aegypti) is responsible for the transmission of dengue to approximately 50 million people each year. Dengue is most common in Africa and Asia and thankfully occurs in only mild to moderate forms, which can cause high fever, severe headaches, and joint and muscle pain, but rarely leads to the death of the infected patient.

Amazingly enough, even though this infectious disease affects some 50 million people annually, there isn’t a specific drug designed to treat dengue fever. The most commonly prescribed medication to reduce the symptoms associated with dengue fever is … Tylenol. That’s right, Johnson & Johnson’s (NYSE:JNJ) Tylenol, whose active ingredient is acetaminophen, works to reduce both fever and muscle pain in patients. In the most serious cases of dengue fever, IVs and blood transfusions may be needed.

As I mentioned previously, estimating new and ongoing cases for some of these diseases can be downright difficult, and perhaps none more so than tuberculosis. TB is caused by a bacteria found in the lungs that can cause chest pain and a bad cough, as well as lead to a number of other nasty side effects. According to WHO, it’s also the second-leading global killer behind AIDS as a single infectious agent.

The majority of TB-associated deaths (95%) occur in low- to middle-income countries where TB awareness and prevention simply aren’t where they need to be. The good news is that TB death rates on a global basis are falling; however, there were still 8.6 million new cases of TB reported last year, and roughly one-third of the world’s population carries a latent form of TB, meaning they’ve been infected but aren’t ill and can’t transmit the disease yet.

While many of today’s TB treatments have long since come off patent, the FDA did approve a new drug in late 2012 named Sirturo, made by Johnson & Johnson, to treat multidrug-resistant TB. Plainly put, not all strains of TB respond to the common forms of treatment, and MDR-TB is an especially virulent killer, so Sirturo has a genuine shot at treating what is essentially the worst of the worst when it comes to TB strains. Peak sales estimates for the drug range between $300 million and $400 million.

Health Conditions & Diseases

  • Cancer
  • Cholesterol
  • Health Care Associated Infection Reporting
  • High Blood Pressure
  • HIV/AIDS Counseling & Testing Sites
  • Hyperthermia
  • Hypothermia
  • Missouri Million Hearts
  • MRSA
  • Obesity
  • Show Me Healthy Women
  • STDs
  • Time Critical Diagnosis System (Stroke and STEMI)
  • Traumatic Brain Injury

Few things impact your life more than a serious health problem. Chronic diseases, including heart disease, stroke and diabetes; cancer; and communicable diseases affect the health of millions of people and cost billions of dollars in medical expenses every year in Missouri.

Lifestyle choices – such as not smoking, maintaining a healthy weight and being physically active – can help prevent some of the most common chronic diseases and some types of cancer. Regular health screenings can often discover chronic conditions and cancers early when treatment is more likely to be successful. Communicable diseases can often be prevented through a number of measures including vaccines and proper hand washing.

By making smart choices, Missourians can avoid or overcome many health problems and live longer and more productive lives.


About two in five Missourians will have cancer during their lifetime. Screenings can detect several types of cancer including breast cancer, cervical cancer and colorectal cancer.

Chronic conditions

Chronic diseases include asthma, heart disease, stroke, diabetes and arthritis. These diseases often can be prevented or controlled keeping risk factors, including high blood pressure, high cholesterol and elevated blood sugar levels, under control.

Communicable diseases

Communicable diseases are caused by germs transmitted through people, animals, surfaces, foods and air.

Influenza, or flu, is a highly contagious viral respiratory illness. Flu can cause a worsening of chronic medical conditions such as heart disease, asthma and diabetes. To help prevent the spread of the flu, a flu shot is recommended every year, especially for people who are at high risk for flu complications.

Other vaccine-preventable diseases include measles, mumps, polio, dyptheria, tetanus, whooping cough, chicken pox, hepatitis A and hepatitis B.

HIV is the virus that causes AIDS. It attacks a person’s immune system, reducing the body’s ability to fight off infections. Efforts to prevent the spread of the virus, counseling and testing individuals who are at high risk, and caring for people living with the disease are key to reducing HIV and AIDS in Missouri.

Zoonotic diseases are illnesses transmitted from animals to humans. West Nile Virus is spread through mosquito bites. Tick bites are responsible for a number of diseases including lyme disease, Rocky Mountain spotted fever, ehrlichiosis, tularemia and Q Fever. Rabies is spread through a bite from an infected mammal, most often a bat. Taking precautions to avoid bites from mosquitoes, ticks and mammals is the best way to prevent the spread of zoonotic diseases.

Food borne illnesses can be prevented through safe food handling practices including proper hand washing, cooking foods to recommended temperatures and storing leftovers properly. To help promote food safety, the Missouri Department of Health and Senior Services and local public health agencies throughout the state regulate more than 30,000 food service and processing facilities and provide training on safe food handling, preparation and sanitation issues.

Tuberculosis is spread through the air from person to person, usually through close day-to-day contact with a person who has TB such as a family member, friend or close co-worker. TB can be treated and cured if prompt medical attention is received.

Tongue Disease 101: Signs, Causes, Types and Treatment

Your tongue might not be that big, but it’s got some major responsibilities. Without your tongue, eating and speaking would be tricky, for example. The condition your of your tongue also affects your ability to breathe and may influence how pleasant (or unpleasant) your breath smells. Sometimes, things can happen to the tongue that affect its ability to do its job well. Tongue disease can take many forms, and although it sometimes occurs as a result of an infection, that’s not the only factor behind it.

Signs of a Tongue Problem

As the National Institutes of Health (NIH) points out, tongue problems can take many forms and can have a variety of symptoms. One common sign of a tongue problem or disease is pain in the area. A person’s ability to taste foods can also be affected when something is wrong with the tongue. The tongue might also swell, change color, or have changes in its texture. In some cases, people with tongue problems have difficulty moving their tongue, which can make speaking and eating difficult. Bad breath is also occasionally a sign of tongue trouble.

Causes of Tongue Disease

A variety of factors can cause a problem with the tongue. The cause of a tongue disorder usually determines how long the problem lasts and how easy it is to treat. For example, tongue infections, caused by bacteria or fungus, usually clear up after a course of antibiotics or antifungal medicines. Problems caused by a nutritional deficiency, such as anemia tongue, usually clear up when the deficiency is resolved. A few other common causes of tongue problems and disease include dietary choices, cancer, nerve damage, autoimmune disorders, trauma to the tongue (like biting it) and hormonal changes.

Types of Tongue Disease

A few common tongue problems include:

  • Thrush. A type of yeast infection, thrush leads to the development of bumpy white patches on the tongue.
  • Burning mouth syndrome. The exact cause of burning mouth syndrome, which creates a burning sensation on the tongue and other areas of the mouth, isn’t known. It might be caused by nerve damage, allergies, nutritional deficiencies or hormonal changes.
  • Black hairy tongue. Black hairy tongue is usually more of a cosmetic problem than a medical one. People with the condition don’t shed the dead tongue cells from the top of the tongue, leading to buildup, according to the Mayo Clinic. After a while, the tongue looks like it has a coating of dark hair on top. This issue can develop after a person takes a course of antibiotics or as a result of a diet made up of soft foods that don’t scrub the surface of the tongue.
  • Oral cancer. Some types of oral cancers develop on the tongue. Symptoms of tongue cancer can include pain in the tongue, a spot that forms on the tongue and difficulty moving the tongue or jaw.
  • Glossitis. Glossitis is swelling of the tongue. In some cases, it is a sign of another tongue problem, such as thrush. In other cases, it’s its own issue. Geographic tongue, which the NIHnotes makes the surface of the tongue look like that of a map, is an example of glossitis.

Treating Tongue Disease

Treatment for tongue diseases depends on the type of problem and its cause. For example, oral thrush is usually treated with an antifungal medication, often in liquid form. Correcting a nutritional problem or changing your diet might help treat burning mouth syndrome. Black hairy tongue can be remedied with improved oral hygiene, such as by brushing with a Colgate Triple Action toothbrush, which has a tongue cleaner to remove odor-causing bacteria. Treating cancer of the tongue depends in part on the size of the tumor. In some cases, surgically removing the cancer is possible. In other instances, treatment with radiation or chemotherapy might also be recommended.

If you suspect that you are having a tongue problem, your best bet is to schedule an appointment with your dentist for an exam and diagnosis. They will look at your tongue and take a swab or culture to determine if the issue is caused by an infection or something else. Keeping your tongue clean will help you achieve an overall healthy mouth.

Infectious Diseases

What are infectious diseases?

Infectious diseases can be caused by many pathogens, including bacteria, viruses, fungi, and parasites that may cause illness and disease. For humans, transmission of pathogens may occur in a variety of ways: spread from person-to-person by direct contact, water or foodborne illness or aerosolization of infected particles in the environment and through insects (mosquitoes) and ticks.

Signs and symptoms and treatment of infectious diseases depend on the host and the pathogen.

Who is most at risk for getting infectious diseases?

Anyone can get an infectious disease. People with a compromised immune system (an immune system that doesn’t work at full strength) have greater risk for certain types of infections. Those at higher risk include:

  • People with suppressed immune systems, such as those going through cancer treatment or who have recently had an organ transplant
  • Those who are unvaccinated against common infectious diseases
  • Healthcare workers
  • People traveling to at-risk areas where they may be exposed to mosquitoes that carry pathogens such as malaria, dengue virus and Zika viruses.

How common are infectious diseases?

Infectious diseases are extremely common worldwide. Some infectious diseases strike more often than others. For instance, in the United States, 1 out of every 5 people is infected with the influenza (flu) virus each year.

What complications are associated with infectious diseases?

Many infectious diseases cause complications. These can range from mild to severe. For some conditions, complications may include wheezing, skin rash, or extreme fatigue. Mild complications usually disappear as the infection resolves.

Certain infectious diseases may cause cancer. These include hepatitis B and C (liver cancer), and human papillomavirus (HPV) (cervical cancer).

What are the symptoms of infectious diseases?

Symptoms of infectious disease are particular to the type of disease. For example, symptoms of influenza include:

  • Fever
  • Chills
  • Congestion
  • Fatigue
  • Muscle aches and headache

Other infectious diseases, such as Shigella, cause more serious symptoms, including:

  • Bloody diarrhea
  • Vomiting
  • Fever
  • Dehydration (lack of fluid)
  • Shock

You may experience one or several symptoms of an infectious disease. It’s important to see a doctor if you have any chronic (ongoing) symptoms or symptoms that get worse over time.

What causes infectious diseases?

Infectious diseases in humans are caused by microorganisms including:

  • Viruses that invade and multiply inside healthy cells
  • Bacteria, or small, single-celled organisms capable of causing disease
  • Fungi, which include many different kinds of fungus
  • Parasites, which are organisms that live inside host bodies causing sickness

Infectious diseases spread in multiple ways. In many cases, direct contact with a sick individual, either by skin-to-skin contact (including sexual contact) or by touching something another person touches, transmits the disease into a new host. Contact with body fluids, such as blood and saliva, also spreads infectious diseases.

Some diseases spread through droplets discharged from a sick person’s body when they cough or sneeze. These droplets linger in the air for a short period of time, landing on a healthy person’s skin or inhaled into their lungs.

In some cases, infectious diseases travel through the air for long periods of time in small particles. Healthy people inhale these particles and later become sick. Only certain diseases spread with airborne transmission, including tuberculosis and the rubella virus.

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