A communicable disease is carried by microorganisms and transmitted through people, animals, surfaces, foods, or air. Communicable diseases rely on fluid exchange, contaminated substances, or close contact to travel from an infected carrier to a healthy individual.

The disease might need a blood exchange via an injection, float along a sneeze in a movie theater, or get transmitted through childbirth. We understand their origin. Examples of communicable diseases include herpes, malaria, mumps, HIV/AIDS, influenza, chicken pox, ringworm, and whooping cough. Cancer, on the other hand, is not a communicable disease.

Parasites, bacteria, and viruses all qualify as pathogens, nicknamed "germs," and can cause a communicable disease. Their method of transmission, period of dormancy, ease of contagiousness, and relative danger can differ drastically from one disease to the next. Governmental health agencies spend a great deal of time and money studying the risk or spread of various contagious diseases in order to identify outbreaks, prevent reoccurrences, or develop treatments. They compile statistics such as incidence, which measures how many new cases are diagnosed per year, and prevalence, which identifies how many cases exist at any one time.

The study of a communicable disease may intersect with issues of sewage systems, weather patterns, vaccinations, available medical care, public education, travel restrictions, water purity, and social interactions. Health officials work to inform the public on high-risk epidemics, such as one strain of flu or a new variety of herpes. Then they can reduce their exposure with preventative measures such as avoiding risky behaviors or getting vaccinated. Methods of transmission are usually well known, and can therefore be circumvented.

A human-to-human communicable disease could be passed through blood, mucus, uterine fluid, breast milk, semen, saliva, or breath. Often, these are prevented by safer sex, frequent hand washing, proper disposal of waste, etc., but might be accidentally acquired by touching a doorknob someone else has used. Animal-to-human transmission includes rabies acquired from a dog or squirrel bite, malaria from mosquitoes, and Lyme's Disease from fleas and ticks. Other kinds of transmission rely on a contaminated surface or substance. This might be improperly cooked meat causing salmonella poisoning or impure water causing dysentery.