The term odynophagia refers to a non-specific medical symptom (pain when swallowing), and would be neither singular nor plural. It uses singular verbs. (e.g. His odynophagia is not severe.)
Some medical conditions have multiple forms and have usable plurals (e.g. phobias)
Vita is singular. The plural form of vita is vitae
"Is" is the singular form of the verb "to be," used with singular subjects. "Are" is the plural form used with plural subjects.
It is singular. The plural is ellipses.
This is singular. These is the plural form.
'These' is the plural form of 'this'.
The singular form is Dennis. The plural form is Dennises.
The plural form of a singular woman is women.
The singular noun is mass. The plural form is masses.
The noun quantum is the singular form; the plural is quanta.
The plural form for the singular, subjective he or she is they.The plural form for the singular, subjective it is they.The plural for the singular, objective it is them.And even though you didn't ask, I will throw in for no extra charge:The plural form for the singular, objective him or her is them.
singular = status plural = statuses or stati
The singular form is it, the plural form is they(subjective) and them (objective).