Microbes that invade and multiply can disrupt normal function. An infection is the invasion and growth of microorganisms—bacteria, viruses, fungi, or parasites—in body tissues, usually provoking an immune response and often causing clinical illness.

Infections may be localized or systemic, acute or chronic, mild or life-threatening. Transmission routes include droplets, contact, blood, food, water, and vectors. Host factors such as age, immunity, and barriers determine susceptibility and severity.

Diagnosis uses clinical pattern, microscopy, culture, antigen detection, and molecular tests. Treatment targets the pathogen when possible and supports the host. Infection prevention—hand hygiene, vaccination, sterile technique, and safe water—remains more powerful than any single drug.

• Invasion and multiplication of microorganisms in tissues
• Provokes immune response and often illness
• Spread by varied routes depending on the agent
• Prevented by hygiene, vaccines, and control measures