Pathogens

Pathogens are organisms, frequently microorganisms, or components of these organisms, that cause disease. Microbial pathogens include various species of bacteria, viruses, and protozoa. Many diseases caused by microbial pathogens.

A disease is any condition caused by the presence of an invading organism, or a toxic component, that damages the host. In humans, diseases can be caused by the growth of microorganisms such as bacteria, viruses, and protozoa. Bacterial growth, however, is not mandatory to cause disease. For example, some bacterial pathogens cause disease by virtue of a toxic component of the bacterial cell such as lipopolysaccharide. Finally, the damaging symptoms of a disease can be the result of the attempts by the host's immune system to rid the body of the invader. One example is the immune related damage caused to the lungs of those afflicted with cystic fibrosis, as the body unsuccessfully attempts to eradicate the chronic infections caused by Pseudomonas aeruginosa.

Not all pathogens cause diseases that have the same severity of symptoms. For example, an infection with the influenza virus can cause the short term aches and fever that are hallmarks of the flu, or can cause more dire symptoms, depending on the type of virus that causes the infection. Bacteria also vary in the damage caused. For example, the ingestion of food contaminated with Salmonella enteritica causes intestinal upset. But, consumption of Escherichia coli O157:H7 causes a severe disease, which can permanently damage the kidneys and which can even be fatal.