Infectious diseases are the invasion of a [[host (biology)|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 र viruses, though a number of other microorganisms, including some kinds of [[fungi]] र [[protozoa|protozoa,]] also cause रोग. [[Prions]] are borderline, र [[meme]]s would not usually be considered in this scope. An infectious रोग 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 रोग 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, र other damaging symptoms, can be more devastating than the direct damage caused by the microbe.
=== Agents र vectors ===
Infectious रोग requires an ''agent'' र a ''mode of transmission'' (or ''[[Vector (biology)|vector]]''). A good example is [[malaria]], which is caused by Plasmodial [[parasite]]s, 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]] र [[tuberculosis]]).
