A microbiologist studies microscopic forms of life.
microbiology
The person who studies the way of life is called a paleontologist.
A biologist studies all life forms.
I'm assuming you meant "What scientists study all forms of life?" The answer is biologists (the root word "bio" means "life").
biologist
scientists
marine biologist
scientist
Possibly an anthropologist.
A virus
Ecologist
someone that studies alien life forms.
ecologist