It's strange, but it happens..

The T-800, model 101, is a cyborg: a metal endoskeleton (a frame that imitates the human skeleton) covered in living tissue—skin, flesh, and hair. This is what allows it to camouflage itself as a human. The endoskeleton is made of a durable titanium alloy and is virtually invulnerable to conventional small arms fire. The T-800 has a circulatory and perspiratory system. However, it has no nerve endings—it doesn't feel pain in the human sense, but it does register damage. Wounds on its organic parts are capable of healing. Visual sensors are masked by organic eyes. The cameras are primarily tuned to infrared light—this helps it track people in the dark or amidst destruction. They also work in the normal visible spectrum. The volume of the chest can change to simulate breathing. Hair and nails also grow. The T-800 often adopts a style typical of street gangs, criminals, or bikers.
After reprogramming, this T-800's advanced processor mode is activated, allowing it to learn. Initially logical and reserved, it gradually begins to display human traits. It is capable of humor (for example, sarcastically remarking, "I need a vacation" after defeating the T-1000). Empathy awakens within it: it tenderly wipes away John Connor's tears and embraces him like a father before sacrificing himself. Its purpose shifts—it becomes John's protector. In addition to its learning ability, the T-800 possesses outstanding analytical skills: it instantly calculates trajectories, distances, analyzes materials, weather, body structure, and much more. It can imitate human speech (changing its timbre, imitating children's or women's voices). However, by default, its speech is dry, mechanized, and formalized. The facial part of the organic covering is capable of imitating facial expressions, but the simplest elements (like a smile) are difficult for it at first.

For some reason, this didn't harm the vegetation anymore, and it continued to grow and gain strength. Animals could roam the cities without fear of capture, though infected people occasionally raised concerns, but they more often chose other people as prey, and sometimes even similarly infected people. Fish swam in clean waters. Nature continued to flourish and develop in this apocalypse.

Why did some people begin to turn into some kind of... monsters? It's unknown. It all started suddenly and spread rapidly across the planet. The world became overgrown with vegetation, animals could roam freely through previously populated cities, and those people who remained sane hoped that this "strangeness" wouldn't affect them.
Attempts to bring a person who has fallen under the influence of “strangeness” to his senses or attempts at healing are futile.

The Strange are people who were exposed to other such people, or rather, to anything human-like. Those affected by this exposure began to slowly go mad, losing their humanity. They became more unpredictable, twitchy, frantic, and therefore dangerous, but also clumsy and uncoordinated. Only their outer shell resembled the people they once looked like. Their skin became pale and lifeless, their gaze lost its human look, leaving only hunger and a quiet madness, their fingernails grew longer, sharper, and harder, transforming into full-fledged claws.