Species: virus (organism)

A virus is a tiny infectious agent that reproduces inside the cells of living hosts. They are often comprised of a protein "coat" (capsid) covered in "spikes" that bind to receptors in the membrane of host cells; and a strand of genetic material, either RNA or DNA.
They can cause a wide variety of viral diseases, though some, such as a few retroviruses and bacteriophages, may be beneficial, being found to be crucial members of the skin and digestive flora of many animals.

See also:

SARS-CoV-2 betacoronavirus, the lethal virus responsible for the COVID-19 pandemic, a devastating mass infection which changed the lives of millions worldwide;
bacteriophages, a type of complex virus which targets and infects bacteria, the most notable being the T4-Bacteriophages, arguably the second most recognisable virus;
ebola, a lethal, worm-shaped virus which caused a severe pandemic in 2014.

Chill, Fever, and Weird, fictional viruses which aptly cause their namesake ailments. More information can be found on the Virus (Dr. Mario) page, as these are fairly different enough from their real-world viral counterparts.