There’s the doorkeeper, who lets people in and out of the building. Doorkeepers, rather; there’s more than one. And more than one door, too. Problem is, the doorkeepers won’t turn anyone away. They operate the portal however the people going through want. So viruses break into the building, and the immune system doesn’t eradicate them because the viruses went through the doorkeeper, thus legitimizing the virus’s presence in the building. And you can’t eliminate or replace the doorkeepers, because without them, the doors cannot be operated. So. How do you keep the viruses out while not modifying the doorkeepers’ behavior in any way? You don’t. What you do is, you hire a team of virus killers and station them as guards throughout the building. If they see a virus, they’ll kill it.
Next to the virus guard barracks, there’s a drinking fountain. All of the virus guards use it on the way in and out of the barracks. The water fountain becomes infected with something for some reason. All the virus guards get sick and become insane. They now kill only the doorkeepers and leave the viruses alone.
Security finds out about the crazy virus guards and takes steps to preserve the remaining doorkeepers. In the end, there are three doorkeepers left, which isn’t enough. (And there are zero virus guards, which is the correct amount.) So obviously they’ll need to take on additional doorkeepers. But just anybody can’t be a doorkeeper, so the process to beef up the ranks will be excruciatingly tedious, and it will take a long time, and meanwhile, all these doors need operated. In the end they set it up so that when you get to a door and there’s no doorkeeper, there’s a button you can press. Then, the next available doorkeeper will come and open the door for you. So there’s a bit of a wait. The doorkeepers are given Segways and pagers so they can get to the doors faster, but it’s far from ideal.
Meanwhile, everyone has forgotten about the viruses. What have they done during the time of insufficient doorkeepers? They decide to abandon their quest to infect the building and decide to help out. They learn to be doorkeepers. They learn very quickly, and the doorkeeper ranks are rebuilt (and well-built) in a very short amount of time. It is decreed that viruses are free to go where they wish as long as they promise not to be bad viruses. Virus persecution ends.
This has all happened before.