W. Richard Carnahan, MD
Infectious diseases are caused by microscopic organisms that penetrate the body’s natural barriers and multiply to create symptoms that can range from mild to deadly. Although progress has been made to eradicate or control many infectious diseases, humankind remains vulnerable to a wide array of new and resurgent organisms.
Types of Infection
Some infections, such as measles, malaria, HIV and yellow fever, affect the entire body. Other infections affect only one organ or system of the body. The most frequent local infections, including the common cold, occur in the upper respiratory tract. A serious and usually local infection of the respiratory tract is tuberculosis, which is a problem worldwide.
Other common sites of infection include the digestive tract, the lungs, the reproductive and urinary tracts, the eyes or ears. Local infections can cause serious illnesses if they affect vital organs such as the heart, brain or liver. They also can spread through the blood stream to cause widespread symptoms.
The outcome of any infection depends on the virulence of infectious agents, the number of organisms in the infecting inoculum and the response of the immune system. A compromised immune system, which can result from diseases such as AIDS or treatment of diseases such as cancer, may allow organisms that are ordinarily harmless to proliferate and cause life-threatening illness.
Modes of Infection
Common ways in which infectious agents enter the body are through skin contact, inhalation of airborne microbes, ingestion of contaminated food or water, bites from vectors such as ticks or mosquitoes that carry and transmit organisms, sexual contact and transmission from mothers to their unborn children via the birth canal and placenta.