Loki:

When you're ill, Loki totally freaks out. At the slightest evidence of illness he forces you to go see a doctor, and then uses one of his days out to come and look after you.

Tony:

Tony doesn't like illness. He gets you everything you want and makes JARVIS and a bunch of people wait on you hand and foot, but unless you ask specifically he won't come near you.

Steve:

He looks after you. Want food? He's cooking it. Want medicine? He'll go and get it. Want to watch a movie? He's already putting it on.

Clint:

He's really strict. He makes you go to the doctor, forces details of what you should be doing every minute of the day out of the generally terrified doctor, and then has you follow everything they said to the second. It's kind of annoying, but you know he only does it because he loves you.

Bruce:

He'll abandon his mega important work to run samples and see if he can't get you a cure quicker.

Bucky:

Bucky doesn't know what to do. He gets really worried, even if it's only a tiny illness. Usually, you have to calm him down, and then you make him curl up and watch a movie with you.

Pietro:

He can be very, very annoying, to say the least, constantly joking about it and generally being a pest. But if you want anything, he'll get it for you in seconds.