A virus is a small infectious agent that replicates only inside the living cells of other organisms. It can cause a wide range of diseases in humans, animals, and plants. Viruses consist of genetic material, either DNA or RNA, surrounded by a protein coat called a capsid. Some viruses also have an outer envelope made of lipids.

Viruses are different from other microorganisms like bacteria because they cannot reproduce on their own. They rely on host cells to provide the necessary machinery and resources for replication. Once inside a host cell, a virus takes over the cellular machinery and redirects it to produce more viral particles. These new particles can then infect other cells and continue the cycle of infection.

Viruses can cause a variety of diseases, ranging from the common cold to more severe illnesses like influenza, Ebola, and COVID-19. They can be transmitted through various routes, including respiratory droplets, contact with contaminated surfaces, and insect bites. Some viruses, like HIV, can also be transmitted through sexual contact or sharing needles.

Prevention and control of viral infections often involve measures such as vaccination, practicing good hygiene (e.g., handwashing), using personal protective equipment (e.g., masks), and implementing public health measures (e.g., quarantine and social distancing). Treatment for viral infections typically focuses on relieving symptoms and supporting the body's immune response, as there are limited antiviral drugs available.