Infection is the invasion of a
host organism's
bodily tissues by
disease-causing
organisms, their multiplication, and the reaction of host tissues to these organisms and the
toxins they produce. Infections are caused by microorganisms such as
viruses,
prions,
bacteria, and viroids, and larger organisms like
macroparasites and
fungi.
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. Pharmaceuticals can also help fight infections.
The branch of
medicine that focuses on infections and pathogens is
infectious disease medicine.