And the very Aussie, "I'm feeling crook!" For having a cold, "have the dreaded lurgy" is often used in the UK and Australia. Not sure if it's used in the US? Also, "sick as a dog" is a fairly common simile used for describing illness.

I've got the flu/one heck of a cold/ a wog. I feel like the pits, my throat is killing me, my nose won't stop running, I've used a million tissues, my head feels twice its size.

I'm not feeling good. I'm a little under the weather. I'm sick as a dog. I have the sniffles.

I caught a virus, keep distance please! :D

I am feeling a bit off colour; I feel a little out of sorts ; I am not myself /feeling myself; I feel a bit funny.