Respiratory Aids

In medicine, infectious disease or communicable disease is disease caused by a biological agent such as by a virus, bacterium or parasite. This is contrasted to physical causes, such as burns or chemical ones such as through intoxication. more...

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While the word \"infectious\" can cause alarm, it is simply a term for a whole class of diseases. Infectious diseases need not be dangerous, and may not spread from person to person at all.

Basics

Infectious diseases are the invasion of a host organism by a foreign replicator, generally 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. Prions are borderline, and memes would not usually be considered in this scope. 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.

Agents and vectors

Infectious disease requires an agent and a mode of transmission (or vector). A good example is malaria, which is caused by Plasmodial parasites, chiefly 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).

Mortality from infectious diseases

The World Health Organization collects information on global deaths by International Classification of Disease (ICD) code categories. The following table lists the top infectious disease killers which caused more than 100,000 deaths in 2002 (estimated). 1993 data is also included for comparison.

(Note: The other categories of death are maternal and perinatal conditions (5.2%), nutritional deficiencies (0.9%), noncommunicable conditions (58.8%), and injuries (9.1%).)

The top three single agent disease killers are HIV/AIDS, TB and malaria. From 1993 to 2002, the death ranking for AIDS went up from 7th to 2nd and Hepatitis B went down from 6th to 11th. While the number of deaths has decreased in almost every disease, it has increased four-fold in AIDS.

Read more at Wikipedia.org