Although there are major differences between bacteria, protists, and viruses, there are similarities as well. Types of all three can cause disease. All three have the genetic material common to all living things--DNA, RNA, or both--even through viruses are not currently considered to be living organisms. All have proteins as part of their structures. All three have beneficial aspects for humans. Bacteria are an essential part of our digestive process, as well as having uses in food production for items such as cheese and wine. Protists include photosynthetic types that produce oxygen, and seaweed (kelp) is used as food in various parts of the world. Viruses, because of the specific types of cells they target, are currently being investigated as a way to deliver "good" genes to cells in medical gene therapy.
There are obvious differences between the three. Viruses are not considered to be alive, while bacteria are prokaryotes and protists are eukaryotes. Viruses cannot reproduce without infecting a living cell. Bacteria reproduce asexually by binary fission. Protists reproduce both asexually and sexually.