Infection with a pathogen does not necessarily lead to disease.
Infection occurs when viruses
, or other microbes
enter your body and begin to multiply. Disease occurs when the cells in your body are damaged as a result of infection and signs and symptoms of an illness appear. The incidence of disease among those infected varies greatly depending on the particular pathogen and individual susceptibility.
Many of the symptoms that make a person suffer during an infection—fever, malaise, headache, rash—result from the activities of the immune system trying to eliminate the infection from the body.
In response to infection, your immune system
springs into action. White blood cells
, and other mechanisms go to work to rid your body of the foreign invader. Indeed, many of the symptoms that make a person suffer during an infection—fever, malaise, headache, rash—result from the activities of the immune system trying to eliminate the infection from the body.
Pathogenic microbes challenge the immune system in many ways. Viruses make us sick by killing cells or disrupting cell function. Our bodies often respond with fever (heat inactivates many viruses), with the secretion of a chemical called interferon
(which blocks viruses from reproducing), or by marshaling the immune system’s antibodies and other cells to target the invader. Many bacteria make us sick in the same way that viruses do, but they also have other strategies at their disposal. Sometimes bacteria multiply so rapidly they crowd out host
tissues and disrupt normal function. Sometimes they kill cells and tissues outright. Sometimes they make toxins that can paralyze, destroy cells’ metabolic
machinery, or precipitate a massive immune reaction that is itself toxic.
Other classes of microbes attack the body in different ways:
Trichinella spiralis, the parasitic worm (helminth) that causes trichinosis, enters the body encased in cysts residing in undercooked meat. Pepsin and hydrochloric acid in our bodies help free the larvae in the cysts to enter the small intestine, where they molt, mature, and ultimately produce more larvae that pass through the intestinal wall and into the bloodstream. At that point they are free to reach various organs. Those that reach skeletal muscle cells can survive and form new cysts, thus completing their life cycle.
Histoplasma capsulatum, a fungus that causes histoplasmosis, grows in soil contaminated with bird or bat droppings. Spores of the fungus emerge from disturbed soil and once inhaled into the lungs germinate and transform into budding yeast cells. In its acute phase, the disease causes coughing and flu-like symptoms. Sometimes histoplasmosis affects multiple organ systems and can be fatal unless treated.
that cause malaria
, which are members of the genus Plasmodium, have complex life cycles. Sporozoites, the stage of the parasite that infects new hosts, develop in the salivary glands of Anopheles mosquitos. They leave the mosquito during a blood meal from a human, enter the host’s liver, and multiply. Cells infected with sporozoites eventually burst, releasing another cell form, merozoites, into the bloodstream. These cells infect red blood cells and then rapidly reproduce, destroying the red blood cell hosts and releasing many new merozoites to do further damage. Most merozoites continue to reproduce in this way, but some differentiate into sexual forms (gametocytes) that are taken up by the female mosquito, thus completing the protozoan life cycle.