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 such as the common cold and influenza. Unlike bacteria, viruses cannot survive or reproduce independently; they require a host cell to multiply. Once inside a host, they hijack the cell's machinery to produce more virus particles, which can then spread to other cells. This unique behavior makes viruses a significant focus of study in medicine and virology.