The 20th century will go down in the history of world as one of the most successful period of human development and civilization. The century was marked by improved technological and advancement in many areas that affected the social life of the people. More important, the century marked important advancement in the medical field with a breakthrough in the treatment of a number of diseases which had remained a problem to human health before. However the century also make the rise of a number of disease which crossed the century as a great threat to the existence of human race on this earth. Notably the century saw the emergence of HIV/Aid which has threatened human existence and posed a great challenge to the scientific world. As we stand, there is no cure or any preventive measure like vaccine that has been discovered for this disease. Therefore we can say that as the century opened may medical frontiers that led to discover of cure for many disease, there were new medical challenges that were coming up.