An infection is a condition in which a virus, bacteria, fungus or parasite enters the body and causes a state of disease. These invading organisms (pathogens) interfere with the normal functioning of the host and can lead to chronic wounds, gangrene, loss of an infected limb and even death. Headaches, muscle aches, fever, chills and fatigue are common symptoms of infections. Many of these symptoms are due to inflammation and the response of the immune system to the pathogens.
In general, viral infections are systemic — involving many different parts of the body or more than one body system at the same time (a runny nose, sinus congestion, cough, body aches, etc). They can sometimes be local as in "pink eye" and herpes. Only a few viral infections are painful, like herpes.