1. He is sick.

2. He has sickness.
3. He has sick.
4. He has a sickness.
5. He has a sick.

(Hello. I have a question. #1 is correct, but what about the others? Which ones are correct?)

1 2 and 4 are correct, but only #1 would normally be used.

If one is going to say someone has a ___ , the missing word should be a specific ailment, like "He has a bad cough"

The forth one is correct and that is it. The other examples are extremely grammatically incorrect.

Hello! Allow me to explain the correct usage of the given sentences related to sickness:

1. "He is sick." - This sentence is correct and commonly used. It means that the person mentioned is currently ill or not feeling well.

2. "He has sickness." - This sentence is not commonly used in English. Instead, one would typically say "He has a sickness" or "He has an illness." However, it's important to note that "sickness" and "illness" are more general terms, and it would be better to specify the specific sickness or illness.

3. "He has sick." - This sentence is grammatically incorrect. In English, "sick" is usually used as an adjective to describe someone's state of being, rather than as a noun on its own.

4. "He has a sickness." - This sentence is grammatically correct. It means that the person mentioned is afflicted by a specific illness or disease.

5. "He has a sick." - This sentence is not grammatically correct. "Sick" is typically used as an adjective and requires a noun to modify it. Without a noun, the sentence becomes incomplete.

Therefore, the correct sentences are numbers 1 and 4: "He is sick" and "He has a sickness."