Microorganisms capable of causing disease—or pathogens—usually enter our bodies through the eyes, mouth, nose, or urogenital openings, or through wounds or bites that breach the skin barrier. Organisms can spread, or be transmitted, by several routes.

Contact: Some diseases spread via direct contact with infected skin, mucous membranes, or body fluids. Diseases transmitted this way include cold sores (herpes simplex virus type 1) and sexually transmitted diseases such as AIDS. Pathogens can also be spread by indirect contact when an infected person touches a surface such as a doorknob, countertop, or faucet handle, leaving behind microbes that are then transferred to another person who touches that surface and then touches his or her eye(s), mouth, or nose. Droplets spread by sneezes, coughs, or simply talking can transmit infection if one person comes in contact with the mucous membranes of the eye(s), mouth, or nose of another person. Influenza is spread by airborne droplet transmission but more commonly by indirect contact on surfaces.

 

Common vehicles: Contaminated food, water, blood, or other vehicles may spread pathogens. Microorganisms like E. coli and Salmonella enter the digestive system in this manner.

 

Vectors: Creatures such as fleas, mites, and ticks—called vectors—can also transmit disease. The most common vector for human infection is the mosquito, which transmits malaria, West Nile virus, chikungunya, dengue fever, yellow fever, and Zika.

 

Airborne transmission: Pathogens can also spread when residue from evaporated droplets or dust particles containing microorganisms is suspended in air for long periods of time. Diseases spread by airborne transmission include tuberculosis, measles, hantavirus pulmonary syndrome, and Legionnaires’ disease.