loanwords - Why is ホームシック a noun, not a な-adjective?

In class, I was told that ホームシック is a noun, not a な-adjective.

ホームシックの時、何をしますか。

By contrast, the closest English word to "ホームシック", which is "homesick", is an adjective.

Why is ホームシック a noun? Is it describing a disease (if you had to use English, you might say "homesickness"), rather than the state of having the disease?