Infectious disease: Encyclopedia II - Infectious disease - Basics

Infectious disease - Basics

Infectious diseases are mostly caused by tiny microorganisms, often called microbes, that are invisible to the naked eye. Microbes that cause illness are also known as pathogens. The most common pathogens are various bacteria and viruses, though a number of other microorganisms, including some kinds of fungi and protozoa, also cause disease. An infectious disease is termed contagious if it is easily transmitted from one person to another.

An organism that a microbe infects is known as the host for that microbe. In the human host, a microorganism causes disease by either disrupting a vital body process or stimulating the immune system to mount a defensive reaction. An immune response against a pathogen, which can include a high fever, inflammation, and other damaging symptoms, can be more devastating than the direct damage caused by the microbe.

Infectious disease - Agents and vectors

Infectious disease requires an agent and a mode of transmission (or vector). A good example is malaria, which is mainly caused by the parasite Plasmodium falciparum but does not affect humans unless the vector, the Anopheles mosquito, is around to introduce the parasite into the human bloodstream.

The vector does not have to be biological. Many infectious diseases are transmitted by droplets which enter the airway (e.g. common cold and tuberculosis).

Adapted from the Wikipedia article "Basics", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki