PTC is not contagious like the flu, but it does affect everybody that is around the person with PTC. PTC can cause crippling headaches, blindness, and various other problems. If a person with PTC is having a bad day (painful day) then they may not be able to function as they regularly would so others may have to help them out or even do stuff for them. There is no select type of person who gets PTC, or what causes it.
Hopefully this helps you understand it better.
Myself (person with PTC)