Why "got a sore throat" but "got - hepatitis"?

I'm learning english grammar and I found some sentences that made me a bit confused.

Why "got a sore throat" but "got - hepatitis" ?

How to use "got a something" and "got something" ?

Thanks.

The general rule is that you do not use either the definite, or indefinite article with the names of diseases or medical conditions.

"Hepatitis" is the name of a medical condition.

"A sore throat" is not the name of a medical condition, it is a description of a symptom. A sore throat could be caused by a medical condition such as streptococcal pharyngitis (commonly referred to as 'strep throat).

So it is correct to say:

I have a sore throat (symptom)
I have strep throat (medical condition)
I have a fever (symptom)
I have hepatitis (medical condition)

There are a few notable exceptions - "the flu" being the most common one. This is just a historic common name for influenza and has become idiomatic. Other historic names for medical conditions such as "the plague" or "the black death" are still referred to with the definite article, along with some fictitious "conditions" such as "the dreaded lurgy" or " the heebie-jeebies".

Further exceptions exist with "compound" words such as "toothache" or "headache". Idiomatically, most people say "I have a headache", but "I have toothache" (no article). The links to the NHS website (United Kingdom's national health service) also show these used in this way. These could be viewed as descriptions of symptoms, but also as "names" for what you are experiencing, so you should check examples to see how they are idiomatically used.

These exceptions are not an exhaustive list.