Orphan drug
An orphan drug is any drug developed under the 1983 U.S. Orphan Drug Act, which concerns drugs for orphan diseases, namely such affecting less than 200,000 people in the US. This has been adopted as a subclause of the FDA. Developing a drug for groups that small would be financially unsound. Therefore, development of drugs for such diseases is rewarded by tax reductions and a monopoly for that drug for a limited time (7 years). Under that premise, many drugs have been developed, included among them are drugs to treat AIDS, cystic fibrosis, and snake venom.
Here is a link to the Orphan Drug Act at the FDA