It does seem that neuritis is not a common word. I'm not sure whether I'd encountered it before, though its meaning is easy to guess: inflammation of the nerves.
Looking at the Google ngram viewer and the COHA corpus, it appears that the word was more frequently used many years ago - around the end of the 19th and beginning of the 20th centuries (ngram) or in the 1920s and 1930s (COHA). Has the condition become less common? Probably not. Since many different ailments can cause inflammation of the nerves - auto-immune conditions, infectious diseases, physical injury - the term neuritis might not be used by a medico explaining a patient's condition. There may be people who have suffered from neuritis without ever knowing the word.
I'll make neuritis a rare word. For your information, the plural can be either neuritises or neuritides. Both words are accepted in Chi, and have always been, and will remain, rare.