What are microparasites?
A parasite is a living organism that depends upon another living organism – for example, a human or animal host – to survive. A microparasite is a subcategory of parasite that is short-lived, smaller than other parasites, and reproduces within its host, in contrast to macroparasites, which do not reproduce in their hosts. [See www.parasiteecology.wordpress.com/2013/04/19/microparasite-vs-macroparasite/] Examples of microparasites include malaria, influenza and hepatitis.
While short-lived, microparasites are defined in part by the fact that they do follow a life-cycle that ends in death but that, as noted, includes reproductive cycles that enable the type of microparasite to survive in its host, if not each individual parasite. According to Britain’s Royal Society, microparasites include various types of viruses, bacteria, protozoans, and fungi, and the life cycles of some types of microparasites can range from five to 12 years, and that the lengthier life cycles “are driven by the host-parasite interaction.” [See “The Population Dyamics of Microparasites and Their Invertebrate Hosts,” Philosophical Transactions of The Royal Society, www.rstb.royalsocietypublishing.org]
The significance of mircoparasites today involves their increasing tendency to evolve into ever-more virulent forms that are resistant to antibiotics. Microparasites that fall into this category include measles, tuberculosis, gonorrhea, and meningitis.
"A microparasite is a parasite that completes a full life cycle within one host and can be transmitted directly to conspecific hosts."
In other words, it is small enough to live and die inside it's host organism AND infect other hosts directly.
In contrast, parasites like tapeworms complete some of their life cycle outside of their host, and cannot go directly from host to host.