evidence: Viruses are biological entities that can only thrive and multiply in a host, which is a living organism such as a human, an animal, or a plant. Some viruses cause disease. For example, severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2, causes the disease COVID-19. A virus may also affect one organism in one way but a different one in another. This explains why a virus that causes illness in a cat may not affect a human. [...] Viruses are biological entities that are present in all living beings. Some are harmless, while others can cause a range of diseases, from the common cold to Ebola.