A microscopic, single celled organism, can be either animal cell like or have chloroplasts and more plant cell like.
Bacteria are: Microscopic single celled organisms, they don't have a nucleus. They have a circular chromosome of DNA. Some can photosynthesise. Most bacteria feed off of other organisms.
Viruses are particles, rather than cells. They are smaller than bacteria. They can only reproduce inside living organisms, this makes viruses parasites.
Organisms as Pathogens.
Some organisms are Pathogens, Pathogens are organisms that cause disease.
That can include some fungi, Protoctists, bacteria and viruses.