All the white blood cells are voiced by black actors.
For bonus brilliance, they're white blood cells in a white person (Frank is Caucasian).
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 than 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).
The designs of the different microorganisms and cells are distinct from one another that you can tell which of the characters are human cells based on how anthropomorphic they are. Human cells as depicted in the movie are not only more visibly anthropomorphic (i.e., they are the most human-shaped) but are also uniquely colored to appear to have a translucent outline to their bodies while having a solid internal color, which vary between cell types (for instance, red for red blood cells, blue-purple for neurons, and cyan for white blood cells). Humans, as animals, do not have cell walls. Microorganisms, meanwhile, have more variety to their appearances—with multiple legs, eyes, and mouths—and are opaque with darker outlines.
If every cell in your body is alive and sentient, minor injuries in the world of Osmosis Jones become horrifying. Not to mention the fate of the poor blood cells carried off by mosquitoes and other biting bugs...
Frank will die one day, and when he does the cell civilization inside him will be destroyed. Now, imagine if everyone else had a civilization of cells living inside them...
Probably, but cells don't live too long (except for brain cells) so it's more like the end of the world after many, many generations. The more dramatic case would be the death of an infant causing a newly formed world to expire too soon.
Also, everybody is basically controlled by a "mayor" and multiple other body cells inside the head. Basically, nobody actually has any free will...
But we are controlled by multiple cells inside of our heads.
If it's any consolation, in the animated spin-off, The City of Hector's mayor deliberately cranks up his phobia of water (gotten after he almost drowned) to keep him out of the pool. Hector overcomes this anyway, so there is some leeway.
When you think about it, a sentient virus would pride themselves 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).
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.
The pilot to Ozzy & Drix had them confronting another Red Death-like virus before getting carried off to Hector, so it's possible that Thrax may have already done it offscreen.
However, the villain in that episode was directly identified as Scarlet Fever, which is real and is a disease, not a virus. That said, the similarities suggest that Red Death may be a fictitious "cousin" to Scarlet Fever.
On that note, are all the potential Thraxes out there just as depraved and evil as he is? Are some of them Reluctant Psychos who don't want to hurt anyone but know they have to in order to survive (since that's basically how viruses work)? Either way, it's not a pleasant thought...
The way Thrax is hellbent on killing as many humans as possible, as quickly as possible, it could be a class 1 or 2 for the outside world if he escaped Frank.
When Thrax died in the beaker of alcohol, that's what happens to every germ on your hands whenever you use hand sanitizer. Yikes.
Because of Aesop Amnesia, in Ozzy & Drix, Frank for some reason starts becoming unhealthy again, and both Ozzy and Drix are carried away by a mosquito to someone else...
Considering Shane is absent and Ozzy talks about getting promoted out of "snot patrol" (when he should have something much higher after the events of the movie), it's possible this is an alternate universe.
There is another more grim explanation for Frank's declining health. Frank's internal temperature nearly reached 108 during the events of the film, which is deadly. However, if one's internal temperature reaches 104, which Frank's surpassed, that can result in permanent brain damage. This puts a dark contrast on the situation; Frank tried to get healthy but resulting brain damage from his brush with death meant he kept forgetting (from the anthropmorphised cell POV, the brain cell in charge of memory could've been killed and the records or information cataloguing system destroyed) to maintain his new healthy lifestyle or he just discarded the attempts.
Thrax claims that, once he breaks his record by killing Frank in two days, he plans to go after Shane and kill her in one day. We see from her initial reaction when Frank flatlines that Shane would not be taking Frank's death well at all. Considering grief can weaken the immune system(which, in the context of the movie, could have been represented as the police becoming increasingly apathetic according to Shane's ongoing mood), Shane would have been a very easy target if Thrax attacked straight away.
The DNA beads Thrax carries around are each trophies from his earlier successful infections; each representative of a person he's killed. There are over twenty beads in that chain. Now, figure in that Thrax killing the human also kills every single cell inside them — which are established as sentient in this universe — and that factors into possibly one of the highest body counts of an animated villain period.