The infected are people who have a disease called Oripathy.
Think of it as rock cancer.
You get it by inhaling too much originium particles in the air, or by introducing large amounts of originium to the body. Think of originium as magic tiberium. Every living thing on Terra, save for the Abyssal Hunters, havr a little bit of originium in their blood. Not enough to be lethal or for it to start converting your cells into more originium, but it's something that can be measured.
Oripathy, contrary to popular belief, can't be spread by making contact with the Infected. Instead, it mainly spreads due to improper handling and protection of workers who mine or refine originium. That, and when a Infected dies, their entire body turns into the stuff and then dissolves into tiny particles. The disease has a incredibly varied list of symptoms as its effects will mainly depend on where it begins to form. It can be anywhere from your brain to your skin to your nerves.
People who suffer from Oripathy are feared due to this, and the fact that since they have significant amounts of Originium in their bodies, they can use it as a power source to cast magic (Called Arts) at the cost of advancing their disease and shortening their life. Mind you, we know that a simple magic missile can be compared to a RPG or hand grenade, so the infected can be seen as always armed even whe most of them don't know how to use magic in the first place.