Which statement best defines an infection?

• A. A pathogen invades and begins growing within a host.
• B. A pathogen remains on the skin surface without entering tissues.
• C. A harmless organism that does not invade tissues.
• D. An infection is caused only by viruses.

Answer: (A)

Infection means a pathogen not only enters the body but also multiplies inside, invading tissues and often triggering an immune response and symptoms. This growth within the host is what differentiates an infection from mere exposure or harmless presence. The other scenarios describe situations that don’t involve tissue invasion and replication: a pathogen that stays on the surface without entering tissues isn’t causing an infection; a harmless organism that doesn’t invade tissues isn’t disease-causing; and infections can be caused by many types of pathogens, not just viruses. So, the statement that a pathogen invades and begins growing within a host best defines infection.