As you may recall, a virus consists of altered DNA housed in a protein shell. The specific construction of the protein shell was really a form of chemical logic which allowed it to attach to a living cell, defeat it's protective mechanisms, and insert the DNA it contained into the cell. The DNA could then alter the cell's genetic code. It also contained instructions that would force the cell to produce copies of the virus, with both the DNA and the protein shell.
This protein logic proved quite complex. The developer had thought ahead quite well and determined that it would be wasteful for a virus to attack a cell that had already received the content DNA. So he provided a chemical signal the shell would generate to alert other viruses that this cell had already been commandeered. Each virus also had the capacity to detect this signal and to wait to attach until this signal was not detected. In this way these genetic modification modules, as we called them, could alter every cell in an organism's body in a matter of days.
One day, in a meeting the squad had with the boss, a member of the sea animal section reported on experiments he had performed involving some alterations to the virus. These alterations specifically addressed the difficulty of getting the cell to release the copies of the virus it had produced.
If the cell died, he suggested, its structures would be reabsorbed by the organism's body, and the viruses it contained would be free to roam the body seeking new cells to commandeer. The DNA inside would be modified to instruct the cell to produce hundreds or even thousands of copies, thus speeding the progress of the virus through the body.
You've probably already figured out the major flaw in the plan. Every cell into which the altered DNA was introduced would die, and the genetic modifications would not become part of the organism, or of a new species. Someone brought this up right away.
The developer had a plan to overcome this obstacle; an elaborate plan which he had worked out in theory but not in practice. The introduced genetic codes would instruct the cell to divide before it died so that the altered DNA would be part of the new cell. The division would also have to arrange it so that the new cell was not infested with the virus but did carry the chemical "lockout" signal that prevented other copies of the virus from attacking it. Once all cells had been thus altered, the viruses would find no more cells to attack and would eventually break down and vanish.
We were all startled when the boss interrupted and ordered that a new squad be formed to develop some uses he had in mind for the current form of the virus. Only a few from our squad joined the new squad, the rest were taken from other assignments. The rest of us (minus the developer since he joined the new squad) would be free to work on the cell division issue.
What possible use could the boss have devised for this rather useless form of the genetic modification module, we wondered. We didn't have to wait long to find out. The new section released new viruses within weeks. The effects of these new devices left many of us sick with disgust. Most of us believed our goal was to improve life on your planet. It became clear, however, that the boss intended, with these viruses, to cause suffering and death.
As we completed other tasks assigned to the squad, more and more of us were assigned to the new virus section. Eventually, even I was put to work at this distasteful job. Some of these viruses caused minor malfunctions of a body's systems, usually accompanied with pain or some other discomfort. Others brought death, some sooner than others.
Of course, the viruses that brought quick death rarely had time to spread to other organisms before their host died. This meant that it was hard to infect a large population or; as the boss preferred; to introduce a virus and have it reproduce and spread perpetually. We quickly learned that our work would be wasted unless we provided a mechanism for our viruses to spread from one organism to another.
Allow me to brag a bit here. When I first joined the virus section, the boss commanded (he never asks) me to develop more means for spreading the viruses. The section had already developed viruses that spread through the interchange of bodily fluids during sexual intercourse. (The boss expressed great delight over this development. I'm not entirely sure why this is, but I think it has to do with his failed first plan or with the original disagreements he had with the master. It's all very complex and I couldn't explain it here even if I understood it all.)
I already knew that most larger bioorganisms had systems to remove foreign material from the respiratory passages. Anything that irritated the delicate tissues of these passages would generate the response. The bodies usually encapsulated the irritant in a slimy substance and forced it out with violent expulsions of air through the passages. If I could get a virus to generate this response, then it would be contained in the material expelled during the process. By building a hardy protein shell, I had a virus that spread easily from one organism to the next.
Each new virus must, of course, be originally introduced into a population. For years your primitive science allowed us to operate in your world with little fear of discovery. (Granted, the master often impedes our work; though I don't understand why, when he has the power to do so, he doesn't stop everything we do.) We could introduce a virus in just about any location and none of you would catch on to our involvement.
In the last century your science has advanced rapidly. (I and many others suspect the master's involvement in this rapid advance.) There are parts of the world where we dare not introduce a new virus because our involvement would be too obvious to you. In some less developed areas, however, we can introduce a virus and have time for it to become part of the natural scene before your scientists figure out that it's there.
One recent example is a particularly nasty device introduced in the continent you call Africa. This virus attacks the very cells in your bodies designed to protect you from such invasions. Not only does this weaken the system to allow the virus to work with less interference, it also makes you more susceptible to other infections, including infections by some of the single-celled organisms we have altered. A person infected with this virus, which you call the human immunodeficiency virus (HIV), will usually suffer a slow but certain death.
By now it really wasn't much of a surprise to us when the boss ordered that this particular virus be set up to spread through the exchange of body fluids, such as that which occurs during intercourse. (Don't make me repeat my little speech here!) We hadn't really planned it, but we soon discovered that people who shared needles when taking intravenous drugs also spread the virus. The boss really liked this because the addictive nature of the drugs in question made it more likely that you would engage in these risky behaviors even when your scientists learned a lot about the virus and issued warnings. (This is part of the reason the boss gives for using sexually transmitted disease organisms and viruses.)
Another means of introducing a virus involves an intermediate step. First, we develop a virus that infects an animal species with which you humans have frequent contact. Of course the very nature of a virus means that it rarely crosses the boundary between species, but we can make that happen.
Once you learn about the animal virus and have become used to its presence, we introduce a specially altered form into the animal species. You won't notice at first, because it has the same effect on the animal (a chicken, for example) as the first version did. But this new form contains protein logic that allows it to attack human cells. Once it has made the crossover the usual means of spreading a virus from human to human take over. The introduction has been accomplished and you have no way of knowing this wasn't just a chance mutation.
Remember, while I may sound like it at times, I have no pride in these accomplishments. Granted, I may feel the occasional twinge of professional accom