The word "semelincident" is pronounced as /sɛməlɪnsɪdənt/. Its spelling is a combination of two different words. Semel meaning "once" in Latin and incident meaning "an event or occurrence." The word implies a one-time occurrence or happening. The unique spelling of this word is due to its Latin roots and the need to accurately represent the pronunciation. As a relatively uncommon word, it's important to spell "semelincident" correctly to avoid confusion or misunderstandings.
Happening once only, said of an infectious disease, one attack of which confers permanent immunity.
A practical medical dictionary. By Stedman, Thomas Lathrop. Published 1920.