Virus

A virus is a small, non-living infectious obligate intracellular parasite that replicates itself inside the living cells of other organisms. Viruses must reproduce by infecting living cells and using the living cells machinery to produce new viruses. These infectious agents can either contain DNA or RNA as their genetic material, and also contain a protein coat, known as capsid. They are also very specific in the cells that they infect, each virus infecting specific cells either infect them through a lytic cycle or lysogenic cycle. According to Kurtzgesagt, a virus is a hull around genetic material.

Bacteriophages
A bacteriophage is a virus that infects bacteria and usually has a helical structure or an icosahedral structure.

Plant Viruses
A plant virus is a virus that infects plants. They usually have a rod/polyhedral shape. The best-known plant virus is the rod-shaped tobacco-mosaic virus.

Animal (Eukaryotic) Viruses
A animal virus, example being: Ebola, is a virus that infects animals. Usually, these viruses have a helical/icosahedral structure and can be "naked" or "envelope". Naked viruses have a protein capsid but no lipid envelope, which "envelope" can mean a lipid envelope that the virus develops when it leaves the host cell.

Retrovirus
A retrovirus is a virus that contains RNA instead of DNA as their genetic material. RNA must be converted back to DNA--once changed, DNA is then inserted into DNA of the host cell. This requires an enzyme known as reverse-transcriptase so that RNA can be copied back to RNA. A good example of a retrovirus is HIV.

Germ Theory of Infectious Diseases
The Germ theory of disease is an accepted scientific theory relating to germs proposed by Italian scientist Girolamo Fracastoro. The theory states that microorganisms are the cause of many diseases.
 * Five Ways to Contract a Disease
 * 1) Viruses
 * 2) Bacteria
 * 3) Protists
 * 4) Fungi
 * 5) Worms
 * 6) Coughing or sneezing near an uninfected person if the person who ejects their mucus is infected.