Infection:The invasion and multiplication of microorganisms such as细菌,viruses, and parasites that are not normally present within the body. Aninfectionmay cause no symptoms and be subclinical, or it may cause symptoms and be clinically apparent. An infection may remain localized, or it may spread through thebloodorlymphaticvessels to becomesystemic(bodywide). Microorganisms that live naturallyinthe body are not considered infections. For example, bacteria that normally live within themouthandintestineare not infections.
