As a noun aphtha
is (pathology) candidiasis, oral thrush, thrush.
As an adjective aphthous is
of, or relating to aphtha.
aphtha
English
Noun
(pathology) candidiasis, oral thrush, thrush
An oral ulcer, disregarding the cause.
foot-and-mouth disease
aphthous
English
Adjective
(head)
of, or relating to aphtha