A virus is an organic particle, even smaller than bacteria, somewhere between a living and non-living cell. Viruses cannot thrive without a live host and must invade a host body to replicate their viral particles. Most viruses are pathogenic, causing disease in their host, including upper respiratory tract infections.
A virus doesn’t have cells. It is made up of a single nucleic acid core (containing genetic material, DNA, or RNA). This is surrounded by a protein coat, and a fatty envelope makes up the outermost layer. Spikes then protrude from the surface and help it attach to host cells.
Between living and non-living particles
Requires a host cell to replicate
Visible under a standard microscope
Viruses can be considered microparasites. Viral particles attach to and invade cells in the host’s body, using the cells’ own process to grow and multiply viral components. Once matured, the new viral particles are released to invade new host cells. Some viruses kill the host cells as part of their life cycle. This can lead to inflammation and unpleasant symptoms in the host.