We create the list of symptoms by looking for health conditions mentioned in web results, and then checking them against high-quality medical information we’ve collected from doctors for our Knowledge Graph. We worked with a team of medical doctors to carefully review the individual symptom information, and experts at Harvard Medical School and Mayo Clinic evaluated related conditions for a representative sample of searches to help improve the lists we show.

Source : Official Google Blog: I’m Feeling Yucky 🙁 Searching for symptoms on Google