Check your employer's sick policy. It's pretty common that they ask you to specify on self-certification forms, and if you get a sick note from your doctor, that'll give sme indication. But consider just how vague you can be! At my work place, we have to pick from a dropdown list which includes things like "virus" (could be a cold sore, could be Ebola), "musculoskeletal"(stubbed toe? Slipped disc? Fractured skull?) or, my personal favourite, "substance abuse" (who admits to that one???)
If its the return to work interview where your manager gets a bit inappropriate, try just saying, "sorry, but it's a bit personal. I'd really rather not talk about it." As I see it, a return to work interview should really be an opportunity for your employer to assure themselves that you are ready to get back to it, and to make sure they are doing what they need to to prevent you getting sick again. If you think it's dissolving into gossip, then a gentle hint should be enough to remind them what you're there for.
Do keep in mind that it might not be in your interest to be too obscure about it, and it almost certainly isn't in your interest to lie about it. Ok, they don't need the gory details of a one-off stomach bug, but say you're suffering from recurring back trouble, or chest infections, your employer ought to be given fair warning that they might need to improve your workstation or the air quality.