Francisco has a good answer. I especially agree w/ the Inbound strategy.

Put yourself in their shoes and predict what they're thinking/wondering/asking about. Create a content publishing strategy around those topics/questions.

The forums you mention will be a goldmine. Read the discussions and look for patterns in the language these patients use. Make sure your publishing matches their language.

Look for patterns in the patient profile, too.

Are they all likely to be 65+? Likely to be overweight? Likely to be male? Likely to be white/caucasian? Etc etc. If you can identify a persona, it will make ad targeting easier.

For example, 80% of mesothelioma patients are male. 95% are white. Knowing that, you could safely advertise to white men with a degree of confidence in your targeting.

The last strategy you may want to consider is a social media-focused 'shareable' campaign. Write/create/produce some piece of content about your cause with a strong TAKE OUR SURVEY call-to-action. (A poignant, touching, emotional video comes to mind, but I suppose it depends on the condition in question.)

The further the video spreads online, the more exposure your survey receives.