Question to the Department of Health and Social Care:

To ask the Secretary of State for Health, what estimate he has made of the number of known conditions that meet NHS England's definition of an ultra-rare disease; and how many people in England are known to have been diagnosed with such diseases.

In the United Kingdom, the term “ultra-rare disease” is generally used to describe a condition that affects fewer than 500 people, although this is not a formal or legal definition. This equates to a prevalence of about 1 person in 100,000.

We have been informed by NHS England that no formal estimate has been made of the number of known conditions that meet this definition or those diagnosed. However, the Orphanet directory of rare disease lists about 250 diseases with a prevalence of 1 in 100,000.