Viruses are tiny infectious agents that can only replicate inside the living cells of an organism. They are composed of genetic material, either DNA or RNA, surrounded by a protein coat. Viruses can infect a wide range of hosts, including animals, plants, and even bacteria, leading to various diseases. Some well-known viruses include influenza, HIV, and COVID-19. Unlike bacteria, viruses cannot survive or reproduce independently; they require a host cell to multiply. Once inside a host, a virus hijacks the cell's machinery to produce more virus particles, which can then spread to other cells. This process can cause significant harm to the host, leading to symptoms of illness.