Viruses are complicated. Depending upon who you’re talking to, they may be defined either as extremely simple microorganisms or as extremely complex molecules. No matter which way you define them however, there is no doubt that they are the causative agents of various nasty infectious diseases. Each individual viral particle consists of a protein coat (or capsid) surrounding an RNA or DNA core of genetic material. This genetic material provides the blueprints which allow the virus to reproduce itself once it finds a host cell. The capsids may also be surrounded by a fatty “envelope.” Viruses with an envelope tend to disintegrate faster and not live as long outside the host animal. Viruses (such as parvo) that are nonenveloped can survive for much longer outside their host.

The ultimate goal of every virus is to make as many copies of itself as quickly as possible. In order to accomplish this, the viral particles must enter a living cell. It accomplishes this by using its capsid proteins to attach itself to the outside of a cell. Once this has been accomplished, the viral particle either injects its genetic material into the cell or enters the cell whole, capsid and all. Either way, the virus uses the cell’s built in machinery to replicate its own genetic material over and over again. New virus particles escape the confines of the cell either by “budding” out from the cells surface a few at a time, or by crowding so many particles inside the cell walls that the cell itself bursts and distributes viral particles throughout its surroundings. In this manner the virus infects more and more cells and spreads throughout the body.

	Each individual viral critter is referred to as a viral particle. The term ‘particle’ is used because it’s a bit of a stretch to call one an ‘organism’ due to their extremely small size and somewhat un-lifelike characteristics.
	The nucleic acids (DNA or RNA) of each viral particle are surrounded by a protein coat and sometimes a fatty envelope. This protein coat and envelope are together referred to as the capsid. The capsids are an extremely important element of the viral particle, as their proteins determine to which cells the virus can attach and infect.

How does a virus go about infecting an animal?

Put simply, each viral particle needs to locate a host cell and inject its nucleic acids inside the cell. This is where the capsid proteins are important – they determine not only what species the virus can infect (cat, dog, pig, human and so forth) but also what cell type within the host it can infect. Common viral targets include blood cells, intestinal cells, and brain cells. Once the particle has bound to a specific cell, it injects its DNA or RNA into the formerly healthy cell. Thinking the new information is its own, the cell proceeds to make copy upon copy of the virus until it either dies or explodes. Either way, the virus uses the cell’s built in machinery to replicate its own genetic material over and over again. New virus particles escape the confines of the cell either by “budding” out from the cells surface a few at a time, or by crowding so many particles inside the cell walls that the cell itself bursts and distributes viral particles throughout its surroundings. In this manner the virus infects more and more cells and spreads throughout the body.