tertian
Concise Oxford English Dictionary © 2008 Oxford University Press:
tertian /ˈtəːʃ(ə)n/
▶adjective Medicine denoting a form of malaria causing a fever that recurs every second day.
– origin ME (fever) terciane, from L. (febris) tertiana, from tertius ‘third’ (the fever recurring every third day by inclusive reckoning).
'tertian' also found in these Oxford entries:

