If you mean disorders in the human, physicians (especially with a Ph.D.) often look into those.
The name for this type of scientist is a teuthologist
A scientist who studies animal life is a zoologist. There is no particular name for a scientist that specialises in platypuses.
The type of scientist that studies the environment is an enviromentologist.
what scientist studies hydrogen and oxygen
A botanist.
An Entomologist
Sedimentologist.
A zoologist.
paleontologist
Meteorologists
a mineralogist
Cosmologist