What Is an Infectious Disease?

By Katharine Paljug @kpaljug
September 25, 2017

Learn about what an infectious disease is, common types of infectious diseases, and how infectious diseases are spread. Read more here.

What is an infectious disease?

An infectious disease is an illness that can spread between individuals.

Infectious diseases are caused by microorganisms, or pathogens, that disrupt your body’s normal processes. A microorganism can be a bacterium that causes bacterial infections; a virus that causes viral infections; a fungus that causes fungal infections; or a parasite that causes parasitic infections.

Infectious diseases can be mild or deadly. Depending on the type of pathogen, they can cause temporary infections that last a few days or weeks or permanent infections that stay with you the rest of your life.

The Centers for Disease Control and Prevention (CDC) reports that, in the United States alone, infectious diseases cause nearly 4 million hospital visits and nearly 20 million doctor’s visits annually. Those numbers only count reported cases; many people with infections never seek medical care.


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How are infectious diseases spread?

How an infectious disease is spread depends on the type of disease.

Many pathogens are spread directly from person to person. Bacteria and viruses are often spread through direct contact, by eating or drinking something that a sick person has touched first, or by using something that an infected person has already used. Many childhood illness, from colds to measles, are spread through person to person contact.

Some viruses can be spread only through direct contact with another person’s bodily fluids, such as saliva, mucus, or blood. Sexually transmitted diseases, for example, are caused by contact with an infected person’s genitals or sexual fluids.

Animals and insects can also spread infections to people. Mosquitos, for example, can carry malaria and infect people that they bite. Other infectious diseases are spread through contaminated food or water. E. Coli, a bacterium that lives in feces, causes a severe gastrointestinal infection that is often spread through unclean drinking water.

Vaccines can prevent the spread of some infectious diseases. These medications contain small amounts of a pathogen, which causes your body to protect itself against future infection. When enough people are vaccinated, a disease can no longer spread from person to person and disappears from a geographic region. This is known as “community immunity” or “herd immunity.”

Smallpox, for example, one of the deadliest diseases in history, was eradicated by a vaccination campaign led by the World Health Organization (WHO). The last known natural case of smallpox was in 1977 in Somalia.

Types of infectious diseases

The most common infectious disease is the common cold. This is a viral infection of the nose and throat that causes symptoms like a runny nose, sneezing, and coughing. It is spread through direct contact with the virus or through using items that an infected person has touched. Colds are not usually serious, and symptoms generally resolve within a few days.

The second most common infectious disease is influenza, or the flu. Like a cold, flu is a viral infection, and it attacks the lungs, nose, and throat. It causes many of the same symptoms as a cold, as well as fever, chills, muscle aches, and fatigue. There are many types of flu viruses, and each year a new vaccine targets the flu strains that are expected to be most common.

Other common infectious diseases include strep throat, urinary tract infection, ear infection, pneumonia, and sexually transmitted diseases. These are caused by pathogens in a specific part of the body. For example, strep throat is caused by the bacteria streptococcus attacking the throat, while pneumonia happens when the lungs are infected with either a virus or bacteria.

There are thousands of types of infectious diseases, some more serious than others. The National Institute of Allergy and Infectious Diseases has a database of infectious diseases.

Are infectious diseases dangerous?

Though many infectious diseases will resolve with time or, in the case of bacterial infections, antibiotics, some can become deadly. This is particularly true for high-risk groups, including the elderly, young children, those with weakened immune systems, or those who are already ill.

An example of this is influenza. A healthy adult who comes down with the flu will usually recover within a few days. But flu, according to the CDC, also causes hundreds of thousands of hospitalizations every year, as well as tens of thousands of deaths, in the United States alone.

Other infectious diseases are always dangerous.

HIV/AIDS, for example, is spread through sexual contact with an infected person, by sharing needles for tattoos or drugs, or from a pregnant woman to her unborn child. Though it can be controlled with treatment, it cannot be cured, and without treatment it is fatal. The WHO estimated that HIV/AIDS caused 1.1 million deaths in 2015 alone, and more than 36.7 million people live with the disease on a daily basis.

Diseases like Ebola, by contrast, often occur in fatal outbreaks and spread rapidly through a geographic region. An Ebola epidemic in 2014 and 2015 affected several countries in West Africa, killing tens of thousands of people. The disease was spread through person to person contact and through objects, such as bedding, that infected people used.

You can protect yourself against infectious diseases by receiving vaccines, washing your hands, cleaning your home and workspace, properly preparing food, and avoiding areas with disease outbreaks. Your doctor can also suggest healthy ways to protect yourself and your family from disease.


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March 26, 2020

Reviewed By:  

Janet O’Dell, RN