An infectious disease is an illness caused by a microbial germ, or microscopic organism. The four main sources of infections associated with infectious disease are bacteria, viruses, fungi and parasites.

An infection is when germs infest and multiply in the body. The immune system fights these germs and is sometimes able to cure the infection. However, if the immune system is weakened or is unfamiliar with the invading germs, it may not be as effective, and the body experiences disease caused by the germs as a result. Symptoms of infection vary widely between infectious diseases, but they can vary from fever and fatigue to organ failure.

Infectious diseases, also called communicable diseases, can spread between people, between animals, between animals and people (this is called a zoonotic disease), and between the environment and animals or people. Contagious diseases are ones that spread through direct contact with infected individuals or their secretions (such as saliva or mucus). Infectious diseases can also spread through indirect contact, like getting bitten by a mosquito that previously bit an infected person or touching a surface that an infected person touched. Some pathogens are spread through the air on particles or droplets in the breath.

There are many ways that germs can invade the body, including through contaminated food, water or medical devices; through wounds or bites; through sexual contact; and through the eyes, nose or mouth. Simple measures such as cooking food thoroughly, promptly cleaning and treating wounds, practicing safe sex, and washing hands frequently can help prevent infection. Masks and physical distancing can reduce the spread of many airborne pathogens, such as SARS-CoV-2. Getting vaccinated can also significantly decrease the chance of contracting an infectious disease.

Examples of infectious diseases still common in the U.S. are the various hepatitis viruses, influenza, Lyme disease, AIDS, COVID-19, shingles and syphilis. The world’s deadliest infectious diseases are either spread by mosquitoes, such as malaria, or are lower respiratory infections, which is a broad category that includes COVID-19, pneumonia and tuberculosis.

These terms are often confused. A virus is a type of germ that can invade the body and cause an infection. A disease is an illness — caused by the germ itself and/or the immune response to it — that happens after the body is infected. One way to think of it is like the difference between a pencil and a drawing. The pencil marks the paper, and the drawing is the result of those marks.