28 Days Later is a zombie movie that doesn't contain any zombies
The film follows a man named Jim who got into a bike accident and fell into a coma for about 4 weeks (hence 28 Days) and wakes up to find London pretty much empty.
Or Maybe It Just Always Looks Like That
Of course then he quickly discovers it isn't as red eyed crazed individuals try to beat the shit out of him. He is rescued, teams up with some people, goes from place to place, see's more dead people than Haley Joel Osment, and finally takes down some military people who are generally being pricks.
We learn early on that the virus was spread by activists trying to free some monkeys from a lab, thus proving that environmental activists do nothing good for the world
Other than giving us something to look at while we eat at KFC
It seems some scientists were experimenting on animals and invented something called the "Rage Virus", which essentially makes them go batshit crazy on anyone who isn't them (see: Republicans). The virus is spread through pretty much any means of bodily fluid from saliva to blood to seamen and it quickly causes you to freak right out and try to kill anyone around you.
Many people have misunderstood this transformation as them becomming zombies. However, there are a few key facts that make them not so.
On the left: Really Pissed Off 28 Days Later People. On the right: Zombies
This last point is the key to the real difference. Starting from its origins in Voodoo, Zombies have always had one thing in common. They died. This is continued in popular culture with the most famous example being that George A. Romero created the "Of The DEAD" series.
"Night Of The Living People Who Want To Beat You To Death" just doesn't have the same ring to it
Dead Rising, Left 4 Dead, Dead Snow, Shaun of the Dead, etc. They all have the theme of death. Even things without "Dead" in the title (such as Resident Evil) require a death to occur before a zombie is created. Without death, there can be no zombie and there lies the problem. In 28 Days Later there are several scenes of perfectly living individuals suddenly becoming infected without ever dropping. All it takes is a simple transfer of fluid and you change. From a simple kiss to a drop of blood to the eye, the virus takes over.
So what is happening to the people in 28 Days Later if they aren't being zombified? Quite simply, getting pissed off. The "Rage Virus" is quite aptly named as it quickly causes the infected to become quite angry quite quick. Either way, they are simply not zombies.