Viral infections are caused by viruses—microscopic infectious agents that invade living cells and hijack their machinery to replicate. Unlike bacteria, viruses are not considered alive outside a host and require living cells to multiply. They can infect any part of the body, from the respiratory system (e.g., influenza, COVID-19) to the gastrointestinal tract (e.g., norovirus), liver (e.g., hepatitis), skin (e.g., herpes simplex), and nervous system (e.g., rabies, viral meningitis). The spread of viruses occurs through various routes, including airborne droplets, direct contact, contaminated food and water, insect bites, or body fluids. Symptoms can range from mild (fever, fatigue, cough) to severe, life-threatening complications depending on the virus and the immune status of the host. Some viruses cause acute infections, while others remain dormant and can reactivate later. Prevention and control of viral infections rely heavily on vaccines, good hygiene practices, and early diagnosis. Antiviral drugs are available for some viruses, but many infections must simply run their course with supportive care. Advances in molecular diagnostics and immunology have revolutionized our understanding of viral behavior, pathogenesis, and immune response—offering hope for more targeted therapies and global health preparedness.