The names of illnesses are usually uncountable in English. Examples are: measles, flu, chickenpox etc.
- Measles is highly infectious. (NOT Measles are highly infectious.)
- There is a lot of flu around at the moment.
The words for some minor ailments are countable. Examples are: a cold, a sore throat, a headache etc.
- I have got a headache. (NOT I have got headache.)
In British English, some common ailments like toothache, earache, stomach-ache and backache are usually uncountable. In American English, they can be countable or uncountable.
- I have got bad toothache. (GB)
- I have got a bad toothache. / I have got bad toothache. (US)
Some uncountable nouns are plural. They have no singular forms with the same meaning and cannot be used with numbers. Examples are: groceries, arms, remains, goods, clothes, customs, thanks, regards, police etc.
- Have you bought the groceries? (NOT Have you bought the grocery?) (NOT Have you bought a grocery?)
- Thanks for your help. (NOT Thank for your help.)
Other plural uncountable nouns include trousers, jeans, pajamas, pants, scissors, spectacles, glasses etc.
- The scissors are in the drawer. (The scissor is in the drawer.) (NOT The scissors is in the drawer.)
The expressions the British, the English, the Spanish, the French, the Irish etc are also plural.
- The Irish are very proud of their sense of humor. (NOT The Irish is very proud …)