HYPOCHONDRIAC

hy ‧ po ‧ chon ‧ dri ‧ ac /ˌhaɪpəˈkɒndriæk $ -ˈkɑːn-/ BrE AmE noun [countable]

someone who always worries about their health and thinks they may be ill, even when they are really not ill

—hypochondriac adjective