A virus is a tiny infectious agent that can only replicate inside the living cells of an organism. Viruses can infect various types of life forms, including animals, plants, and even bacteria. They are responsible for many diseases, such as the common cold, influenza, and COVID-19. Unlike bacteria, viruses cannot be treated with antibiotics and often require vaccines for prevention. Viral infections can spread through different means, including direct contact, airborne transmission, or contaminated surfaces. Once inside a host, a virus hijacks the host's cellular machinery to reproduce, which can lead to cell damage and illness. Understanding how viruses work is crucial for developing effective treatments and vaccines.