Rare  (Merriam-Webster)
Etymology: Middle English, from Latin rarus
Date: 14th century
1 : marked by wide separation of component particles THIN <rare air>

As awareness of spontaneous coronary artery dissection (SCAD) grows, and more patients (some say survivors) surface, it may actually be appropriate to equate SCAD with the concept of “rare air.”

When on a single day, a woman in the South Pacific on the island of Vanuatu and another in the Netherlands raise their virtual hands on the Internet to be heard as SCAD survivors, it does in fact seem that we are needles in a haystack … individuals marked by wide separation.

Social media lets us shake down that haystack and draw our “component particles” close together. And it feels great to be a growing group involved in the search for answers through the Mayo Clinic SCAD virtual registry and DNA biobank.

From NPR, “Patients Find Each Other Online to Jump-Start Research”

Also, the Philadelphia Inquirer, http://articles.philly.com/2012-06-26/news/32409745_1_heart-attacks-citizen-scientists-rare-condition