A viral infection occurs when a virus enters the body, invades healthy cells, and uses the cellular machinery to replicate, often damaging or destroying the infected cells. Viral infections can affect virtually any part of the body, including the respiratory tract, digestive system, skin, blood, or nervous system. Symptoms vary depending on the virus and site of infection but commonly include fever, fatigue, muscle aches, cough, runny nose, sore throat, rashes, or gastrointestinal disturbances. Some viral infections are mild and self-limiting, while others can lead to serious or chronic illness. Because viruses rely on host cells to reproduce, infections cannot be treated with antibiotics, and the severity and course depend on the virus type, the individual’s immune response, and overall health.