Natsu appears as innocent and frail. He doesn't want to sick to other people's business because he only thinks of them as troubles. Like the others, he has an illness which is yet to be revealed.

Spoiler

His illness is a pattern that appears upon his skin when his body temperature rises.This pattern can be transferred to another person but only because Natsu's body heat is also transferred to that person. Natsu takes medicine to keep his body temperature down.

(Source: Wikipedia)