Scientific interest into the microscopic world resulted in a dramatic change of medicine known as Germ Theory, the identification that some diseases were caused by microbes or micro-organisms.
At the most basic levels all living things can be divided into two major groups of organisms, depending on the type of cells they are made of. Those which contain more complex cells and contain nuclei are called eukaryotes (means true kernel or nuclei) and are found in organisms such as animals, plants, fungi and protists.
The other group is less complex, do not have a nucleus and are known as prokaryotes or bacteria.
The most studied are fungi, protozoa, algae, bacteria, archaea and viruses.