How do viruses primarily cause illness in the body?

Viruses primarily cause illness in the body by invading healthy cells and replicating inside them. When a virus enters a host, it attaches to a specific cell type, typically using surface proteins that bind to receptors on the host cell. Once inside, the virus hijacks the cell's machinery to replicate its genetic material and produce new virus particles. This process often leads to cell damage or death, as the cell becomes overwhelmed with the new viral components.

As cells become infected and subsequently die, the immune system is alerted to the presence of the virus, resulting in inflammation and the activation of immune responses. This immune reaction is what often causes the symptoms of illness associated with viral infections, such as fever, fatigue, and inflammation. The direct invasion and replication within cells are thus fundamental mechanisms by which viruses disrupt normal cellular function and lead to disease.