Not all languages treat nouns in the same way. For example, hair is uncountable in English, but is a plural countable noun in many other languages. Similarly, the names of illnesses are usually uncountable in English and are followed by singular verbs.

Measles is highly contagious. (NOT A measles is highly contagious.) (NOT Measles are highly contagious.)

The names of some minor illnesses, however, are countable and can be used with the indefinite article a/an. Examples are: a cold, a sore throat, a headache etc.

Exceptions

In British English, toothache, earache, stomach-ache and backache are usually uncountable. In American English, these words are generally countable.

Many nouns have both countable and uncountable uses.

Compare:

I need to buy some typing paper. (Paper – uncountable)
He went out to buy a paper. (= newspaper; countable)
Glass can be blown. (Glass – uncountable)
I need a glass of water. (Glass – countable)

Some countable abstract nouns (Example: difference, reason, idea, change, difficulty etc.) can be used uncountably after little, much and other determiners.

Compare:

What are the main differences between a dog and a fox? (Difference – countable)
There isn’t much difference between ‘allow’ and ‘permit’. (Difference – uncountable)

The indefinite article a/an is used with some uncountable nouns when we want to limit their meaning in some way.

I want my kids to have a good education. (NOT I want my kids to have good education.)
We need a translator with a good knowledge of Italian. (NOT We need a translator with good knowledge of Italian.)