evidence: Foreign substances (e.g., bacteria or viruses) in the body that are capable of causing disease. The presence of antigens in the body triggers an immune response, usually the production of antibodies. [...] Pathogens: Organisms (such as bacteria, viruses, parasites and fungi) that cause disease in human beings. [...] Virus: A tiny organism that multiplies within cells and causes diseases such as chickenpox, measles, mumps, rubella, and hepatitis. Viruses are not affected by antibiotics, the drugs used to kill bacteria.