What do Prokaryotes, Protists, and Viruses have in common?
Common traits between organisms often depend on their relationship to each other. In this case, both Prokaryotes and Protists are living organisms with a biological structure and, generally, the ability to replicate themselves using nutrients gathered from their surroundings, while a Virus is technically classified as not "living" as it is simply a protein structure and only capable of self-replication after it invades a living cell. However, there are some similarities between the three.
The most basic similarity is that all three can be harmful to multicellular organisms. Viruses can invade cells and replicate themselves, killing the host cells. Prokaryotes, or Bacteria, are often infectious and cause deadly diseases, including Cholera and Tuberculosis. Protists can be spread by insects or food and are the cause of Malaria. All three cause harm by self-replication, overloading the host cells and/or killing them so they cannot fight back.
Another similarity, related to the first, is the speed of self-replication. While viruses cannot replicate without a host cell, many reproduce very fast after infection. Prokaryotes and Protists both replicate without invasion, so they can spread quickly and efficiently. They are also simple structures, with the Protists being the most complex -- protists have a full protein nucleus -- and the Virus the most simple -- viruses are just a protein strand with instructions to replicate.