What this suffix does
The suffix -itis comes from Greek and means "inflammation of". It combines with the name of an organ or tissue to name the inflammatory disease of that area. From "arthr" (joint) you get "arthritis"; from "append" (appendix) you get "appendicitis"; from "tonsill" (tonsil) you get "tonsillitis".
It appears in very commonly used medical vocabulary: many everyday illnesses are named with -itis.
The Spanish connection: -itis = -itis
This is one of the rare technical suffixes where English and Spanish are literally identical:
arthritis = artritis
appendicitis = apendicitis
tonsillitis = amigdalitis
tendinitis = tendinitis
bronchitis = bronquitis
The only difference is pronunciation: in English the stress falls on the penultimate syllable: ar-THRI-tis, ap-pen-di-CI-tis.
Modern humorous use
In modern informal English, -itis is used humorously to name imaginary conditions or obsessive states:
phonitis = addiction to your phone
meetingitis = too many work meetings
Mondayitis = the Monday blues
These are not real medical terms. But knowing that -itis = "inflammation of / too much of" lets you understand the joke instantly.