There are hundreds of species of harmful elements in our world.

The most powerful of which, affect our skin and bodies on the exterior.

But the weakest, those which cannot penetrate the skin or exterior, are the most deadly.

They harmlessly enter into the body of the host. Once settled, they will start to increase their influence with existing cells. They must befriend the cell first, so as to not attract attention of the immune system too early, and keep a low profile. With the help of the symbiont cells, it can then move on to place its influence in key establishments within the body, even convincing the controllers of the cells, those which are placed to regulate the system, that nothing is wrong.

Once that is done, it can continue to establish an environment suitable for its survival.

The exterior of any system is strong, because its meant to fight out unwanted elements. But get inside, and a myriad of vulnerabilities exist. Successful viruses are those who can manipulate any existing system inside the body, because alone, the virus is not strong enough.