Why does a person become ill after being exposed to a virus?

A person becomes ill after being exposed to a virus because the virus invades and begins to replicate within host cells, disrupting normal cellular functions. This triggers the immune system to respond, leading to inflammation and the release of chemicals that cause symptoms like fever, fatigue, and pain. The combination of cellular damage and the immune response results in the observable signs of illness. Additionally, the severity of the illness can depend on factors such as the individual's immune system, the virulence of the virus, and overall health.