Entering the Human Host
Microorganisms capable of causing disease— or pathogens—usually enter our bodies through the mouth, eyes, nose, or urogenital openings, or through wounds or bites that breach the skin barrier. Organisms can spread, or be transmitted, by several routes.
Droplets spread by sneezes, coughs, or simply talking can transmit disease if they come in contact with mucous membranes of the eye, mouth, or nose of another person.
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, mouth, or nose. Droplets spread by sneezes, coughs, or simply talking can transmit disease if they come in contact with mucous membranes of the eye, mouth, or nose of another person. SARS
, and influenza
are examples of diseases spread by airborne droplet transmission.
Contaminated food, water, blood, or other vehicles may spread pathogens. Microorganisms like E. coli
enter the digestive system in this manner.
Creatures such as fleas, mites, ticks, rats, snails, and dogs—called vectors—can also transmit disease. The most common vector for human infection is the mosquito, which transmits malaria
, West Nile virus
, and yellow fever
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 measles
and hantavirus pulmonary syndrome