Virus is an obligate intracellular parasite which can reproduce only by invading and taking over other cell as they lack the cellular machinery for self reproduction.
A microbe is any living organism that spends its life at a size too tiny to be seen with the naked eye, which include bacteria and archaebacteria, protists, some fungi and even some very tiny animals that are too small to be seen without the aid of a microscope.
Viruses and the recently discovered prions are also considered microbes.