Any etiquette surrounding a minor illness is fairly straightforward. We know to be careful not spread anything contagious, and whenever we're in the company of someone who's under the weather, we offer sympathy: "Sorry to hear you've been sick with the flu. I hope you're feeling better."
A more serious illness calls for a get-well card, flowers, or perhaps a visit. And dealing with a debilitating or terminal illness makes more demands on loved ones and friends, all of whom want to make such difficult days as comfortable for the patient as possible.