It's a movie, folks. Movies take all kinds of artistic liberties with reality or else would be pretty damn dull and uneventful if they stayed within the confines of boring-ass reality.
You've seen Guy Standeven in something because the man was in everything.

I feel the same about Star Wars. Theres no way Vader coukd keep his armour suit SO shiny all the time. Unless a film is hyper realistic it must suck.
And in Aliens. I saw that woman in Ghostbusters 2 and she never explained how she got back to earth.
Oh waitits all make believe!

Just watched the movie and came to score it, happened to see this forum thread A little thought-experiment. We could take something non-bacterial and non-viral like a prion disease which is quite fast and fatal, and take rabies maybe. So the evil prion gets into the blood (through a wound, or through capillaries in mucosa), is moved along in the body by blood, and it makes some proteins in the body to switch to wrong folding formation on contact, almost instantly, and maybe also causes some other chemical reactions in the body. So some of the cells affected are those in the throat or stomach lining hence the vomiting of blood, or maybe the prion contains a molecule that reacts with stomach acid making the acid eat through stomach lining. It also affects some cells in the eyes (small capillaries, or some cells specific to eyes only) and some cells (neurons, astrocytes etc) only found in the brain. It makes the normal proteins to change their folding on contact and the speed is exponential. It only takes some seconds for the first evil prions to reach the brain (and penetrate the brain cells, through capillaries leaking maybe, like in the eyes) and from thereon it is like a runaway nuclear reaction. Like in Creutzfeldt-Jakob disease and with rabies it changes the victim's personality and behavior dramatically. So maybe it destroys (part of) the gray matter and with it most of the higher brain functions (and inhibitions) which makes the victim fallow the most 'primitive' of its urges, like hunger, aggression etc. Also, as with rabies, the victim might attack others as it is the main attack vector of the disease. It might be that since the metabolism of the victim changes (so also emitted pheromones) it might be that it attacks primarily those who are not infected and have normal metabolism. The diseased might be drawn to any sound and movement but to really attack they require the victims to 'smell right' hence they move in a pack but mostly attack the non-infected. People with Heterochromia iridum (different colored eyes) have often two types of genes in their bodies (mosaicism). Some people have immunity against some diseases. A person with mosaicism ie two genotypes might have half their cells vulnerable to a disease and half immune. So when such person gets infected with the evil prion it might be that the disease progresses differently, less destructively - one eye is bloodshot and affected other is not, a part of the brain is affected other parts are fine, only part of the stomach lining is affected (or maybe not at all, some organs may contain only single genotype hence be unaffected) etc. In the long run the disease kills its victims but it still spreads if the symptoms are just right (like rabies), the conditions are right (urbanization) and the diseased stay alive long enough to infect at least one other person

Perhaps it is not the virus itself that causes people to turn, but the virus creates a toxin when it replicates, ala the botulism bacteria. The infected subject is loaded with this toxin and bites a new subject. The subject gets a big dose of this toxin and immediately goes beep the virus then has a new body to replicate in and fill up with toxin.
This doesn't cover someone being infected and then immediately infecting another person though.

Considering that a virus basically alters DNA of the cell in order to get the cell to produce more viruses, and transcription of MRNA is not instantaneous, the cell itself would not be able to transcribe the MRNA quickly enough to create the polypeptides necessary for the viruses. Therefore the laws of DNA and RNA elongation, as well as polypeptide synthesis would prevent a virus from being able to replicate itself as fast as the in this movie.
Transcription in bacteria is faster, which allows bacteriophages to replicate faster than a eukaryotic infecting virus. But still, the rate of infection in this movie is completely ludacris and defies the laws of nature.

From what we know of now it is impossible for a virus to act that quickly, but do we know everything out there in the world currently? This might be a movie but who's to say that a new virus breaks out and does affect everyone so quickly. Anything is possible because we as humans do not know everything.

We also know that the time travel presented in Terminator is complete garbage, but the movie still rocks. The 28 x Later movies are no Terminator, but I can still overlook the speed of the virus when watching the flick.

This is why there is a thing called
suspension of disbelief
.
Yes, we - the audience - know it
isn't
possible for any such virus in real life to do what the virus portrayed in this film does, but why should that make this film any less enjoyable than a film about a robot coming from the future to kill you?
Besides, at least the film tries to give an explanation as to how the virus spreads as quickly as it does by showing us a glimpse of it multiplying at an extremely fast rate, rather than abandoning an explanation.
Scream your last breath

Well, I get sick after watching most American trailers.

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The virus wouldn't even have the time to reach the brain, let alone multiply enough in order to affect a human being.
This is what worries you? Your scientific expertise?
I have never tried heroine but from my experience in doing heroine i believe it takes somewhere between 5 minutes and 3 days to start working, or around immediately.
You see what i did there?
http://www.cracked.com/blog/6-common-movie-arguments-that-are-always-w rong/

MY GOLLY YOU MEAN THIS WASN'T A NONFICTION DOCUMENTARY!?!?!?

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A movie featured something that cant be done in real life?!??!
SAY IT AINT SO!!!!

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And lets not forget Aids Indiana j/k
Yeah, we the sheep still don't get it when it comes to the shady sht our government is up to.
HELP AGGRAVATE THE STATUS QUO, VOTE AGAINST EVERY INCUMBENT YOU SEE ON A BALLOT.

I never put any thought into it but knowing who viruses work there's no way you'd turn in 30 seconds.
But I'm not too bothered by it. As long as a movie or series sticks by it's own fictional logic I'm fine with it. There are points when 28 Weeks Later does not do that and I find that more annoying.