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. Infectious diseases, also known as transmissible diseases or communicable diseases, comprise clinically evident illness (i.e., characteristic medical signs and/or symptoms 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, and prions, microorganisms such as bacteria (bacterial infections), nematodes such as roundworms and pinworms, arthropods such as ticks, mites, fleas, and lice, fungi such as ringworm, and other macroparasites such as tapeworms. 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.
The branch of medicine that focuses on infections and pathogens is infectious disease medicine. Physicians and veterinarians and dentists may use specific pharmaceutical drugs to treat infections (antibiotics, antivirals, antifungals, antiprotozoals, antihelminthics).