Infectious disease
A false-colored electron micrograph shows amalaria sporozoite migrating through the midgutepithelia.
Classification and external resources
ICD-10	A00-B99
ICD-9	001-139
MeSH	D003141

Infection is the invasion of a host organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to these organisms and the toxins they produce.^[1] Infectious diseases, also known as transmissible diseases or communicable diseases, comprise clinically evident illness (i.e., characteristic medical signs and/orsymptoms of disease) resulting from the infection, presence and growth of pathogenic biological agents in an individual host organism.

Infections are caused by infectious agents such as viruses, viroids, andprions, microorganisms such as bacteria, nematodes such asroundworms and pinworms, arthropods such as ticks, mites, fleas, andlice, fungi such as ringworm, and other macroparasites such astapeworms.

Hosts can fight infections using their immune system. Mammalian hosts react to infections with an innate response, often involving inflammation, followed by an adaptive response.^[2]

The branch of medicine that focuses on infections and pathogens isinfectious disease medicine. Physicians and veterinarians may use specific pharmaceutical drugs to treat infections (antibiotics, antivirals,antifungals, antiprotozoals, antihelminthics).

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