Infection occurs when harmful microorganisms, such as bacteria, viruses, or fungi, invade the body and begin to multiply. This can lead to various symptoms, depending on the type of pathogen and the area of the body affected. Common infections include the flu, common cold, and strep throat. The body's immune system typically responds to infections by sending white blood cells to fight off the invaders. In some cases, medical intervention, such as antibiotics for bacterial infections, may be necessary to help the body recover and prevent complications.