My ten cents, for what they're worth:
Definitely go ahead with a romantic twist. This won't usually detract from the story (as long as you don't go too prominent with it); rather it enhances your character, adds an extra dimension to the story, and hooks your readers by another thread. All good things! Two I think would be fine, especially if it's another challenge for the MC. But it depends on how long you're planning -- it may end up being something you have to cut out to keep the word count down.
I'm not sure what to say, but I usually go by the policy "only tell the readers what you absolutely have to" (then, I'm trying to cut down a 110,000 word novel to 100,000 or less. Also, my beta readers do tell me I take their intelligence too much for granted sometimes
) So, in short, I would say leave the disease a mystery if you can get away with it. Definitely get input from beta readers to see if it works.