The invasion of any living organism by disease-causing microorganisms (see pathogen), which proceed to establish themselves, multiply, and produce various symptoms in their host. Pathogens may invade via a wound or (in animals) through the mucous membranes lining the alimentary, respiratory, and reproductive tracts, and may be transmitted by an infected individual, a carrier, or an arthropod vector. Symptoms in animals appear after an initial symptomless incubation period and typically consist of localized inflammation, often with pain and fever. Infections are combated by the body’s natural defences (see immunity). Treatment with drugs (see antibiotics; antiseptic) is effective against most bacterial, fungal, and protist infections; some viral infections respond to antiviral drugs. See also immunization.