Altered humans refers to anyone who began life as a normal human being, but through variable and often unknown processes, found themselves mutated, evolved or otherwise transformed into something other than human. It is a rather blanket term that covers a wide range of creatures, some of which may harbor specific alterations germane towards a particular species, such as werewolves. Others may just wake up one day and shout "Holy shit! I'm butt ugly today!"
Toxic waste always seems to play a large part in altering a human's physiology. In the real world, exposure to toxic waste will just kill your ass, but not in movies. Nope. Odds are, you'll turn into a pus-ugly butt freak by the end of the week. Just ask Melvin Ferd. After falling out of second story window into a vat of nasty goop, he transformed into a hideous superhuman muscular powerhouse that lesser men have now dubbed the Toxic Avenger.