Because the disease name isn’t a plural of scleros.
Sclerosis (from the greek skleros meaning hard + osis meaning a disease) is the stiffening of tissue.
osis meaning a disease
Just as additional info: this is correct for English. In Ancient Greek the suffix -ωσις/-ōsis is wider, basically “plop it on a verb to get a noun for process, action or result”; so it’s a lot like one of English -ing suffixes (the one that makes nouns from verbs). e.g.
- the falling = πτῶσις/ptôsis
- the seizing = ἅλωσῐς/hálōsis
i think they know that. if you pluralized ‘sclerosis’, you’d expect to get ‘scleroses’. just like pluralizing ‘thrombosis’ gets you ‘thromboses’.
Scleroses would translate as “hardening diseases” though. There’s only one disease.
The disease isn’t a plural. Which i already said.
Because that “multiple” doesn’t refer to multiple hardening (σκλήρωσῐς/sklḗrōsis) events, but rather to hardening as something uncountable happening in multiple spots.
It’s roughly “multiple hardening”, note how the lack of a plural doesn’t feel off in this one either.
For reference, in languages showing adjective-noun agreement, the adjective gets the singular (e.g. Portuguese “esclerose múltipla” - the plural would be “escleroses múltiplas”).
