Virulence is a term that refers to a microorganism's or virus's ability to make an organism, such as a human, less fit.  There are several factors that will influence virulence:

1. Adhesion- the bacteria or virus must be able to adhere to the cell it is trying to invade to do harm.

2. Colonization- the bacteria or virus must be able to reproduce within the cell it is infecting; lower colonization rates transfers to less virulence.

3. Invasion- the bacteria or virus must be able to invade the cells of the host it is trying to infect.  Typically, most viruses have this quality at the top of their list of skills, the ability to invade a host cell.

4. Immune response inhibitors- some bacteria and viruses are able to secrete substances that will slow the immune system's response to the foreign invaders, thus allowing them more time to reproduce within the host cells.

5. Toxins- some bacteria and viruses secrete toxins that are poisonous to the cells they are invading, causing illness and even death to the infected cells.  Viruses are notorious in this aspect; they usually kill the cell they are invading.

All these factors combine to give the microorganism or virus it's potency, known as virulence.  Diminished health can result anywhere from minor illness to death of the entire organism.