Introduction
So, what is a virus?
There is no straightforward and simple answer as to what a virus is. A virus is neither alive nor dead (in a certain sense). It contains some fundamental information of itself (Deoxyribonucleic Acid, or in other cases, RNA) which allows it to make copies of itself. This "copying" process can only be done when the virus is in a living cell of some kind and due to the extremely minute size of the puny virus, the DNA or RNA information is not enough for it to duplicate. It has to get the "spare parts" from somewhere and the living host cell offers itself as a wonderful candidate as a productions center for more viruses, which in turn are released to find more cells to infect (Anyway, the so-called "spare parts" are the cell's enzymes and its "machinery").
What do these "thingeys" look like?
A virus consists of these main parts: capsid, core and genetic material (The DNA and/or RNA). The capsid is the outer shell of the virus which envelops the genetic stuff inside the virus. It is actually made of many identical proteins which are fused together to form the capsid as a whole. Occasionally, there will be a protein core around the genetic stuff while in other cases, there will be an additional "skin" on the outside called an envelope. It is a lipid bilayer membrane with proteins embedded in it.
What do they do?
Viruses exist for but one purpose and that is to propagate, to make more of their kind and to take over the world (Well, maybe not that dramatic…)! Most viruses appear to be harmful, except for certain viruses which devote their humble existence to exterminating banes to mankind like bacteria. The reason why viruses are harmful is because their duplication process is a bit "host-unfriendly", causing the "host" (an involuntary host, but a host nonetheless) cell to die eventually. This is because viruses enter cells by attaching themselves to a specific structure on the cell's surface and depending on the virus, it might either enter the cell or it might just inject its own genetic material into the cell. Eventually, the cell is subverted and starts creating viruses by the millions. The new viruses are then released either by budding or lysis. In the case of budding, little virus "bumps" start growing out of the cell and then breaks off totally after a short while ("short" is a subjective term). Lysis is more dramatic as it involves the building up of viruses till a certain point when the cell membrane ruptures and releases the viruses all at the same time.
What do they conquer?
Almost anything! Viruses can infect any living thing from plants, to animals, humans and even bacteria.
So, what's the diff (difference, get it?)?
Viruses vary in every aspect, from size, shape to the organization of genetic material inside them. It is, therefore, not surprising that viruses are classified according to the arrangement and the type of genetic material they possess. But, however diverse they may be, they are all capable of replicating within living cells and spawning viruses perfectly identical to the original virus.
People change, how 'bout viruses?
Accidents are but inevitable during the process of replication and sometimes, mutations occur. They might result in the virus being "sterile" but more often than not, the mutations are actually a turn for the better. Because of this, new strains of viruses have to be identified each year and more vaccines created to protect against these new "super" viruses.
What perimeter defenses are required to protect us from the alien scum?
Firstly, note that when a cell is infected, it secretes substances called interferons. These are proteins and may be divided into different groups, such as alpha, beta and gamma interferons. These interferons interact with nearby cells to make them more resistant to infection by the virus. This resistance is not fool-proof and when it fails, we fall ill. Of course, I am also assuming that our immune system also failed (temporary setback) but it is a matter of time (usually) before we defeat the invaders once again. This says something about the second line of defense, the immune system. The white blood cells go postal and chomp up every hostile adversary in sight and to make the destruction complete, the infected cells are also obliterated. In all but one case, we will start to feel better after a while. Our immune system fails against the HIV virus as it is our very defense force which is under threat of extermination and once the HIV invades and triumphs, other "factions" will attempt to establish a foothold on this "Promised Land". Death is unavoidable from this point on.