Seeing a picture of my little hairless rat, you might think she has a skin problem or her cage mates pulled all her hair out but she's just hairless.
But the fact is some rats will do what is called, barbering to other rats. This means that they will clean them so much and so roughly that they actually pull their hair out. They will even do it to themselves. The most common reasons are mites or other parasites.
You can find my article about barbing below.