Osmosis Jones/WMG

Thrax killed Frank's wife.
This might have been the writers' original idea.
 * But Thrax listed his victims at the beginning, and no one was mentioned that might have fit her description.

Thrax and the Grinch are related.
Seriously, does anyone else see the resemblence? And there's the fact that they're about the same size...

Shortly after the end of the movie, there will be a massive outbreak of the Red Death.
Thrax is self-admittedly contagious, highly lethal, has three kills with listed times, and has easily a dozen hypothalami on his chain. It's doubtful that he never once reproduced in all those previous hosts. Worse, Red Death is an entirely unknown virus, with doctors noting that they had no idea what Frank's symptoms meant in the hospital, and the only known sample of the virus was dissolved in alcohol. Humanity is fucked.
 * I forget what he said about his previous victims, but if someone as unhygenic and unhealthy as Frank can fight it off, most everyone but the very young and the very old should be able to survive. In other words, no worse than the Swine Flu Epidemic.

Mayor Phlemming is a brain tumor.
Hence the bad choices he gives to Frank. I could very well imagine added scenes to the movie where the mayor hides in his office and creates copies of himself to hide under his desk.

Multi-cellular organisms took a different route in this movie.
Many cells and other unicelluar organisms are as intelligent as people. In this universe, every multicellular life-form is the biological equivalent of a giant mech - for the most part, the humans lack true sapience, however the cells have sapience. To the cells of a multicellular organism, harmful bacteria and viruses are the equivalent of vandals.

Thrax is an engineered disease that somehow got loose.
It would certainly explain why his first, last, and only goal was the death of Frank, rather then just spreading himself around (as a normal virus or bacteria would).