ulcerative colitis

This term refers to a specific medical condition and is used almost exclusively in clinical, diagnostic, or patient-care contexts. It carries a heavy, serious connotation as it describes a lifelong chronic illness requiring ongoing medical management.

As a medical diagnosis, the term is treated as an uncountable noun. It does not have a plural form because it refers to a singular disease state rather than a countable object. One does not refer to multiple cases of the disease as "ulcerative colitises."