Retroviruses for Dummies

What is

• a virus? A virus is cause of infection, normally too small to see with ordinary microscopes. A virus is not a cell like a bacterium, but a piece of RNA or DNA, its genome, wrapped up in a protein coat. It can only replicate by invading the cells of living hosts and making them copy it. 
• a retrovirus? A retrovirus is a special type of virus. It is RNA based when it is a free virus, but once it enters a host cell, it produces a DNA version of its genome, a 'provirus', and inserts it into the DNA of the cell. The cell then "reads" the viral DNA resulting in the production of more retroviruses.
• an exogenous retrovirus? Exogenous means outside the host genome. An exogenous retrovirus is a retrovirus that invades your cells from outside. 
• an endogenous retrovirus? Endogenous means within the host genome. An endogenous virus is something you acquire by inheritance - your parents passed it down to you.