The infected have evolved in '28 Years Later' — and they're scarier than ever

• The "28 Days Later" franchise sees society collapsing after a virus escapes a lab and turns people into zombie-like killers.
• In "28 Years Later," while people have adapted to survive, the Rage Virus has evolved.
• Here's how the infected are different in the sequel.

Warning: spoilers ahead for "28 Years Later."

In "28 Years Later," the Rage Virus that turned Britain post-apocalyptic in the original movie has evolved, meaning the survivors have new types of the infected to deal with.

Spike (Alfie Williams), a 12-year-old boy, is the film's protagonist. He lives on the island of Lindisfarne, cut off from the mainland by the tide, which saves the community from the Rage Virus.

His father, Jamie (Aaron Taylor-Johnson), takes him hunting on the mainland as a rite of passage, and Spike learns how dangerous the rest of the country has become after 28 years of the virus spreading. It isn't long before the young boy comes face-to-face with the infected.

"There have been evolutions because we didn't want to just stand still with them," the film's director Danny Boyle told IGN in June, referring to the infected.

"28 Years Later" introduces "Slow Lows" and "Alphas."

On the mainland, Jamie constantly reminds Spike to be vigilant of the infected, and explains that there are "Slow Lows" who crawl on the ground eating bugs.

Slow lows are more bloated than the original infected, and their skin is pale and mottled. While they're easier to kill from a distance, their loud screams draw attention. Interestingly, Jamie chooses not to kill one of the Slow Lows when he realizes she's a little girl, and she flees into the undergrowth when he lets her go.

But the Slow Lows pale in comparison to the "Alphas," who are much bigger and stronger than the other infected. One Alpha, who is nicknamed Samson (Chi Lewis-Parry), shows off his brute strength by ripping a soldier's head off along with his spine with his bare hands.

The Alphas can also withstand getting struck by several arrows without dying, although they can be killed by the large flaming javelins used by the guards on the Lindisfarne gate.

Jamie tells Spike that in specific people, the Rage Virus acts like bodybuilding steroids, which appears to make them invulnerable to minor injuries.

The Alphas make things difficult for Spike and his mother, Isla (Jodie Comer), when they go looking for a mysterious doctor on the mainland and come across one in the ruins of an old train.

Knowing they can't kill an Alpha cranks up the terror in a nail-biting chase sequence.