The best answer is "a" - the overuse of antibiotic. Emerging disease are those whose incidence has increased in the past two decades or threatens to increase in the near future. Such diseases include : 1. Old infections emerging as a result of antimicrobial resistance due to indiscriminate or poor use or over use of antibiotics. 2. New infections resulting from evolution or change of existing organisms. 3. Previously unrecognized infections in areas undergoing ecological transformation 4. Known infections spreading to new geographical areas or population. Antibiotic resistance has been on the rise and is now major concern.