Antibiotic resistance
Antibiotic resistance spreads very quickly, far faster than microbiologists expected. "As long as new drugs keep coming, resistance is not a problem. But there has not been a new class of antibiotics discovered since the 1980s".  Some scientists have developed new antibiotics to combat resistant bacteria.  However, there is a lag-time of about eight years between discovery and possible availability for general use. The process is also extremely costly.