Infectious diseases are caused by pathogenic microorganisms such as bacteria, viruses, parasites or fungi viroids, prions, nematodes such as parasitic roundworms and pinworms, arthropods such as ticks, mites, fleas, and lice, fungi such as ringworm, and other macroparasites such as tapeworms and other helminths. The diseases are spread directly or indirectly from one person to another. Zoonotic diseases are infectious diseases of animals that cause diseases when transmitted to humans. 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. Specific medications used to treat infections include antibiotics, antivirals, antifungals, antiprotozoals, and antihelminthics. This session discusses more about infectious diseases.