An orphan disease is a disease which does not attract very much public attention, research, or funding, typically because it is extremely rare and poorly publicized. Thousands of such diseases can be found worldwide, ranging from extremely rare genetic disorders like Fatal Familial Insomnia to tuberculosis, which doesn't attract attention in industrialized nations due to the low incidence of reported cases. In several nations, efforts have been made to promote research into orphan diseases, with the goal of treating people who suffer from these conditions.
A rare disease may become an orphan disease for two reasons. In the first instance, any disease which afflicts less than 200,000 people is generally considered to be an orphan disease, because there are not enough patients to make research cost-effective. Diseases which are common in the developing world but rare or unheard-of in the industrialized world are also termed orphan diseases, because they fail to attract attention from major pharmaceutical companies.
Many orphan diseases are genetic in nature, which can make them very challenging to study, let alone treat. Others take the form of extremely rare viruses, unusual bacteria, or peculiar allergies, and they may take time to diagnose and write up, so they slip through the cracks for months or years until someone begins to connect multiple incidences of the same condition. Researchers such as epidemiologists are often more in touch with emerging diseases than others, but they cannot attract enough attention to extremely rare diseases to make the general public aware of the issue.
From the point of view of pharmaceutical companies, orphan disease research is a loser. To sink a serious investment of time and energy into a potential treatment for a disease which could only be sold to a limited market is, simply, not worth the time. Some governments have recognized this, and provided funding incentives to pharmaceutical companies which choose to research orphan diseases, in the hopes of attracting more interest in developing treatments for these conditions.
Causes which focus on a specific orphan disease also have trouble getting funding, because they are not heavily publicized. Some charity organizations, recognizing this, have banded together under the general umbrella of rare disease research, rather than trying to get funding for a single disease. Donors often feel more comfortable donating to a blanket organization which will distribute the funding as it sees fit than trying to donate to a multitude of causes. These organizations can also privately sponsor challenges to identify, treat, and potentially prevent or cure specific orphan diseases.