In case of very specific requirements, such as rare diagnosis, there might be no point in spending your time and resources on huge public campaigns. What is more, you might even already have a direct contact to your audience via doctors or other contacts. However, in case you don't have the necessary mailing list or contact person, start with finding and contacting channels that have direct access to the cohort of your interest, for example, patient organizations, clinics, or self-organized social media groups.
In this scenario for most of the time you will not directly communicate with your target audience. Thus, your main task will be to prepare various informative materials for your partners.