A virus is an infectious agent that contains genetic material, either RNA or DNA, with a protein coat around it. Viruses are microscopic, and to multiply, they must invade a host. The host can be a human, plant, animal, or bacteria.
When a virus enters a host cell, it hijacks the cell and begins producing its own genetic material and proteins. It then uses the cellular machinery of the host cell to make copies of itself.
Some viruses, when they infect their host, can cause disease. For example, the influenza virus can cause the flu.
Viruses may also affect different hosts in different ways. This is why a virus that may be dangerous to humans does not affect dogs and cats.
There are also various qualities of viruses, such as:
- their shape and size
- their type of nucleic acid
- the presence of a protective lipid envelope derived from the host cell