More than 300 million people worldwide — or 4% of the population — have a rare disease, a new study finds.

A disease is considered rare when it affects fewer than five in 10,000 people, according to a European definition.

Until now, it's been difficult to gauge how widespread rare diseases are. But a team led by a French research institute has analyzed the scientific literature on thousands of rare diseases and created a database that makes it possible to estimate how many people worldwide have them.

Of the more than 6,000 diseases in the database, 72% are genetic, and 70% start in childhood. In addition, 149 diseases are responsible for 80% of rare disease cases identified worldwide.

Rare cancers, as well as rare diseases caused by infection or poisoning, weren't included.

"Although rare diseases are individual and specific, what they have in common is their rarity, and the consequences which result from that," said project coordinator Ana Rath, a researcher at INSERM in Paris, where the database was developed.

Because of a lack of research, health provider expertise and suitable treatments for rare diseases, these illnesses cause significant suffering to many patients and families worldwide, an INSERM news release noted.

Understanding how many people have rare diseases is key to identifying priorities for health and research efforts, understanding the burden these diseases place on society, properly managing patients and promoting public health policy, according to the researchers.

The findings were published online recently in the European Journal of Human Genetics.

Collectively, rare diseases are actually not so rare, and national and global public health policies are needed to deal with them, the authors concluded.