Which category of infectious agents includes both unicellular and multicellular organisms that can be parasitic?

The category of infectious agents that includes both unicellular and multicellular organisms that can be parasitic is helminths. Helminths are a group of parasitic worms that can be quite complex in their life cycles, often involving multiple hosts. They can exist as multicellular organisms, including flatworms and roundworms.

In contrast, fungi primarily consist of unicellular and multicellular organisms, but they are typically saprophytic, obtaining nutrients from decomposing organic material rather than being parasitic in the traditional sense. Protozoa are generally unicellular organisms and may also be parasitic, but they do not include multicellular forms. Mycoplasmas are even simpler; they are unicellular prokaryotic organisms and are known for being bacterial in nature rather than parasitic in the way helminths are.

Overall, helminths encompass a broader range of living forms and include both unicellular and multicellular parasitic organisms, making them the most fitting answer for the question posed.