The misused plural words "malarias" and "plasmodia" have previously been mentioned in the MalariaWorld Newsletter (the former only in passing). Here, I highlight the usage problem by expanding on the subject.
Considering that "malaria" is the name for a disease, "malarias" (in the plural) is acceptable if one is referring to the DISEASES caused by more than one species of Plasmodium. However, "malarias" is incorrect when reference is being made to the causative PARASITES, i.e. when "malarias" is used to mean more than one species of Plasmodium. Plasmodial parasite species are not "malarias". This mistake has become frequent in the journal literature following publication in 1971 of the book entitled "The Primate Malarias".
It must be emphasized that "Plasmodia", which occasionally appears in publications to indicate more than one species of Plasmodium, is wrong. The italicized name "Plasmodia" does not, in reality, exist. This is despite the fact that to write "Plasmodia" seems correct, especially to people who studied Latin at school. Historically, some classical malarial parasitologists, who are likely to have had Latin as a school subject, certainly made this mistake. That is surprising though, since collectively, they will presumably have acquired a better background concerning taxonomic rules than many modern biologists. On the other hand, gaining of such knowledge will not necessarily have happened in respect of medically qualified malariologists; which might well, to a large extent, explain the past occurrence in publications of the erroneous "Plasmodia" linguistic phenomenon. I'm merely speculating, of course.
Use of the term "plasmodia" in this particular malaria-related sense (as opposed to the correct slime mould-associated "plasmodia" sense), and which word is thus derived from the generic name Plasmodium, is not in fact as logical as it looks. It is incompatible with principles in the International Code of Zoological Nomenclature; glaringly so if the word appears with an upper case "P" and in italics, implying a Latinized taxon. Then it is a blatant, albeit inadvertent, violation of the Code.
Instead of the noun "plasmodia", the following can be written for the plural: "Plasmodium species"; or "Plasmodium spp."; or "species of Plasmodium"; or "plasmodial species".
Note that use of the adjective "plasmodial" is perfectly in order.
For further information, see: