Thrax picking up a bit of DNA from hypothalami as a trophy. In the Real World, some viruses actually pick up bits of host DNA and carry it with them. He also is far more interested in killing then in reproducing and spreading; this means that, like a Real Life super-lethal virus, he wouldn't be able to cause disease in many people (see below).
Fridge Horror: Frank will die one day, and when he does the civilization inside him will be destroyed. Now, imagine if everyone else had a civilization of cells inside them.
Also, everybody is controlled by a "mayor" and multiple other cells inside the head. Basically, nobody actually has any free will.
But we are controlled by multiple cells inside of our heads.
When you think about it, a sentient virus would pride themself on killing everything, so slowly but surely Thrax aims to try and kill every living thing on Earth.
Not really. Viruses in the Real World evolve for two things: to make more of themselves and spread to other hosts. A virus that quickly killed its host would severely hamper its own spread (see: Ebola). In many ways, Thrax actually behaves much more like an engineered bioweapon than like a naturally occurring virus (see Art Major Biology entry). Which would mean that he was designed for killing, and that whoever created him (terrorists, the military, take your pick) has more samples stored away...
According to The Other Wiki, bacillus anthracis is a soil-borne bacteria that can be contracted via ingestion and causes death by invoking cold or flu-like symptoms, such as, say, a fever. Bacillus anthracis. Otherwise known as anthrax. What's even more disturbing? How anthrax was even exposed to the monkey cage at the zoo in the first placed, and how many cases of Thrax are now just waiting to happen...
Thrax may be named for anthrax, but he doesn't cause that disease though. He causes an entirely fictional disease, the Red Death.