The Infected

The Infected are the main antagonists of the 2002 post-apocalyptic horror film, 28 Days Later, the second main antagonists of it's 2007 sequel, 28 Weeks Later, the comic series, and (possibly) the film 28 Months Later. The Infected, as their name suggests, are the former residents of the United Kingdom who have been infected with the highly dangerous and contagious Rage Virus.

The Infection
The Rage Virus (or simply known as The Infection) itself is a new mutated strain of the Ebola Virus mixed with an experimental anger inhibiting drug created by Dr. Clive and Dr. Warren. The Rage Virus is easily spreadable in a massive domino effect from the bodily fluids of infected hosts. The fluids of the infected are so contagious that a single droplet of blood or saliva getting in the body of a primate (like, through a cut, a bite from an infected individual, having blood vomited in their face, or even a simple kiss) is enough to convert a host within an incubation time of ten to twenty seconds. It has the power to completely destroy whatever landmass or island it breaks out on. The time it takes to do this depends on where it breaks out at. It took less than a month to completely devastate the United Kingdom. It seems however, that it is possible for someone who gets infected to try and fight and resist he infection. This may draw out the incubation time to about a full minute; but regardless, they will succumb to the Rage Virus very shortly afterwards.

Symptoms
When someone gets infected, they will begin to feel pain. This pain then turns into server agony. And the victim will begin to spasm uncontrollably while beginning to bleed out of their eyes, nostrils, and mouth. The hosts cough up and vomit copious amounts of blood regularly. The Rage Virus, as it's name suggests, causes the hosts to be filled with mindless, animalistic, uncontrollable, all-consuming rage and hatred for the uninfected and everything around them. The Rage is so extreme that The Infected can no longer hold emotional connections with uninfected friends or family, now viewing them as enemies.

Appearance
The outward appearance of a primate infected has not changed much from before. They do however, have very distinguishing marks. Their irises are a glowing scarlet red color. They also constantly bleed out of their eyes, nose, mouth, and scalps. Strangely, chimpanzees don't appear to bleed like human infected. They still have red irises however.

Powers and Abilities
Enhanced strength: While not quite on a superhuman scale. The hosts of the Rage Virus have much more strength than a regular human. They can easily pound down metal doors and gates, smash through barricades, and easily snap necks, and rip humans to shreds.

Enhanced Durability: The durability of an infected is also increased. They can withstand massive amounts of trauma (even be set on fire and not care) and have a complete disregard for pain and physical injury. One great example of the durability of an Infected is shown when an Infected soldier has his body mutilated by the propellers of Flynn's helicopter; he still continued to pursue the survivors despite the massive injury (albeit noticeably slower than the other Infected).

Enhanced Speed; One of the most dangerous things about the Infected is their enhanced speed and relentlessness when in pursuit of a target. The Infected have roughly two times the speed of a regular human. They can cross hundreds of miles of territory in mere days. Destroying any towns or cities in their wake.

Limitless Stamina: The Infected never appear to get tired. They will run infinitely as long as they have a target in sight.

Slowed Metabolism: An average Infected can last about eight weeks before starving to death. The Metabolism of the Infected appears to be slowed down; enabling them to last much longer than a regular human without food or water.

Infectious fluids: As mentioned above, the fluids of the Infected are highly infectious. They can easily spread the Rage Virus to other primates and activate the Rage Virus's domino powers on the population. The most common methods are through bites, and vomiting blood in an individual's face; getting in their eyes or mouth. The time it takes for someone to transform depends on the amount of fluid the person is exposed to. Examples being:

Private Mailer vomited two mouthfuls of blood on Clifton's face. Clifton laid twitching on the ground for nearly two seconds before rising back up as an Infected, succumbing almost instantly.

Frank got a single drop of fluid in his eye. The Virus did take seconds to infect Frank; and he appeared to be trying to resist the Infection. He was killed before he could fully transform however.

Weakness
If the Infected have any weakness at all, it's that they are so solely focused on killing people and spreading the Infection that they no longer care about self-preservation; leading to their eventual deaths from starvation. They also no longer care about keeping themselves safe and healthy. They make no effort to avoid any kind of physical danger; like gunshots and fire.

Intelligent Infected
There appear to very rare types of Infected that have much higher forms of intelligence than most others. As of now, Donald Harris is currently the only known Infected who possess this certain degree of control over the Rage Virus (although Private Mailer could have also had higher intelligence). It's currently unknown what causes this higher intelligence and higher self-awareness in the Infected. Or if there are or will be anymore Infected with this degree of self-control.

Such displays of higher intellect include:

He escaped Alice's cell with his ID card rather than smash through the door or window.

He seemed to smile after killing Alice. He also appeared to watch sadistically as the Infection spread in the safety room. Interestingly, the infected chimpanzees from the first film began clapping and cheering after the activists started spreading the Infection. This possibly meaning that the symptoms of the Rage Virus aren't as server in animals than it is in humans. And that the chimps and Don appear to retain some kind of sadistic thought processing.

He appeared to target military personnel particularly to reduce their numbers and weaken District 1's defenses.

He also appeared to be focused solely on killing rather than both killing and infecting like other Infected.

He hid for cover when NATO firebombed District 1 in a failed attempt to stop the Infection.

He appeared to be actively stalking his children rather than blindly and mindlessly attacking them on sight.

He ambushed Scarlet and bludgeoned her to death with Doyle's rifle. The other Infected don't use any weapons; just their bare hands.