A virüs is a tiny infectious agent that can only replicate inside the living cells of an organism. They are much smaller than bacteria and can infect a wide range of hosts, including animals, plants, and even bacteria. Viruses consist of genetic material, either DNA or RNA, surrounded by a protein coat. When a virüs enters a host cell, it hijacks the cell's machinery to produce more copies of itself. This process can damage or kill the host cell, leading to various diseases. Common examples of viral infections include the flu, common cold, and COVID-19.