Viruses are composed of genetic material (either DNA or RNA) surrounded by a protein coat. Some viruses also have an outer envelope made of lipids. They are very small, ranging from 20 to 300 nanometers in size.

The replication process of viruses involves attaching to a host cell, penetrating it, and taking over its cellular machinery to reproduce. This process can cause damage to the host cell, leading to symptoms of the infection.

There are many different types of viruses, including influenza, HIV, herpes, and COVID-19. Some viruses can be prevented with vaccines, while others can be treated with antiviral medications.