“lack market currency dealers are a longtime fixture in Sudan, where two decades of trade sanctions have battered the economy, cut off access to Western banks, and empowered those with American bills, who can buy 30 percent more Sudanese pounds on the street than they can when exchanging at the official rate. The trade in fake Yellow Fever certificates is, however, a more recent development, which has sparked concern among international health organizations that see proof of inoculation before travel as a prime means of stymieing the disease’s spread.”
Source: Sudan’s Vaccination Card Black Market – The Daily Beast