What is an Infectious Disease? | Types, Definition, Structure, Function & Facts

What is an Infectious Disease?

An infectious disease is any disease caused by a pathogen (germ) such as a virus, bacteria, parasite, or fungus. Although we mainly discuss infectious diseases in humans on this page, other living organisms such as animals, plants, and microorganisms can all become ill from infectious diseases.

What is Pathogens?

“Pathogen” is the scientific name for “germ”. Infectious diseases caused by pathogens. When your mom tells you to wash your hands because of germs, she wants you to get all the germs out of your hands so they don’t get into your mouth and body. Maybe after reading this you will wash your hands a little more!

Pathogens are tiny organisms (called microorganisms) that enter the body and cause disease. Examples of pathogens are viruses, bacteria, parasites, and fungi.


Different types of pathogens cause different types of diseases. Here are some examples of diseases caused by each type of pathogen:

Viruses – Viruses are extremely small and consist of DNA with a protective layer of proteins. Diseases caused by viruses include the flu, colds, measles, yellow fever, and hepatitis.

Bacteria – Bacteria are small microorganisms. They can cause diseases such as tuberculosis, cholera, typhoid fever and tetanus.

Parasites – Parasites are organisms that live off of their host. Some parasitic diseases include malaria, sleeping sickness, and scabies.

Fungi – Fungi are microorganisms such as yeasts and molds. They can cause diseases such as fungal meningitis, ringworm, and thrush.


Infectious diseases can be transmitted in many ways including:

Food and water contamination – Some diseases are transmitted through contaminated food or water. This can happen in undercooked food or when wastewater enters a water source. Examples of diseases transmitted in this way include E. coli, cholera and salmonella.

Insect bites – Insects such as fleas, mosquitoes and ticks can carry diseases that are transmitted when they come into contact with humans. Insect-borne diseases include malaria (mosquitoes), Lyme disease (mites), and plague (fleas).

Direct contact – The disease is usually spread from person to person through direct contact. This may include shaking hands, sneezing (during the flight), and kissing. Examples include the flu, the common cold, and chickenpox.

Indirect contact – Some pathogens can persist for some time outside the host. They can be found on objects such as doorknobs and phones. Many, but not all, of the same diseases that can be spread by direct contact can also be spread by indirect contact.

Other Types of Diseases

Not all diseases are contagious. Some other types of diseases include:

Autoimmune – An autoimmune disease that occurs when the immune system attacks the body. Examples include type I diabetes and multiple sclerosis.

Lifestyle – Lifestyle diseases are caused by the way people live. They can also be the result of people living longer. Examples include type 2 diabetes, alcoholism, and heart disease caused by high blood pressure.

Disorder – An illness is sometimes called a disorder when a part of the body is not working properly. Usually the disorders are related to the brain and are called psychosis. Examples include eating disorders such as bulimia and mental disorders such as depression.

Heart disease – Heart disease is used to describe anything that is wrong with the cardiovascular system. This includes conditions such as coronary heart disease, high blood pressure, and congenital heart disease.

Medical condition – The term medical condition is used to describe everything from trauma to infectious disease to disorder. An injury such as a concussion would be considered a medical condition.

Interesting Facts about Infectious Disease

Shingles is an infectious disease that can be transmitted from animals to humans.

Not all infections are considered diseases because some infections do not make a person sick.

Not all viruses, bacteria, parasites, and fungi are harmful to humans, but those that cause disease are called pathogens.

More than 160,000 people die each year in the United States from infectious diseases.

A “contagious” disease is a highly contagious infectious disease.

Thank you for your interest
Please leave your details to get the best colleges and free counseling
Thank you for your interest
Please leave your details to get the best colleges and free counseling