minated water infected with human or animal fecal matter can contain many
diseases-causing agents. An example is the bacterium Vibrio cholera,which
causes Asiatic cholera, a disease caused by feces contaminated water where san-
itation is poor. This organism invades the intestines and causes severe vomiting,
diarrhea, abdominal pain, and dehydration.
Disease Transmission
A disease must have a portal of exitfrom the reservoir and a portal of entryto
the infected host. This is how diseases are spread and new cases of infection occur.
Examples of portal of exit are the respiratory tract, digestive tract, urinary tract,
skin, and utero transmission. Diseases can spread by three different modes of trans-
mission: contact transmission, vehicle transmission, and vector transmission.
CONTACT TRANSMISSION
Contact transmissionof a disease-causing agent can either be direct or indirect.
Direct contact transmission occurs from skin-to-skin contact, such as shaking
hands, kissing, sexual contact, or making contact with open sores. Examples of
diseases caused by contact transmission include herpes, syphilis, and staphylo-
coccal infections.
Indirect contact transmissionoccurs when infection is spread through any
nonliving, inanimate object. These contaminated inanimate objects are called
fomites and include bedding, towels, clothing, dishes, utensils, glasses and cups,
diapers, tissues, and even bars of soap. Examples of diseases caused by indirect
contact transmission include the rhinovirus, hepatitis B, and tetanus.
Droplet transmissionis a form of contact transmission that occurs through
sneezing, coughing, and speaking in close contact with an infected individual.
Examples of diseases transmitting in this manner are pneumonia, influenza, the
common cold, and whooping cough.
VEHICLE TRANSMISSION
In vector transmissions, pathogens can be spread through the air and in water,
food products, and body fluids (such as blood and semen). Airborne microor-
ganisms mainly come from animals, plants, water, and soil. These microorgan-
CHAPTER 13 Epidemiology and Disease^201