Rare Infectious Diseases cause outbreaks, epidemics, even pandemics that spread from continent to continent. Rare diseases caused by infectious agents rather than genetic or environmental factors. Some of the diseases are Acanthamoeba keratitis, Progressive vaccinia, Rat-bite fever, etc. Modern medicine and hygiene have given us some control over devastating infectious diseases, even eradicating smallpox, but, for the most part they remain with us, often preying upon the poorest and most vulnerable.Scientists think that smallpox, which causes skin lesions has emerged about 3,000 years ago in India or Egypt before sweeping across continents. The variola virus, which causes smallpox killed as many as a third of those it infected and left others scarred and blinded, according to the World Health Organization. Plague, which is caused by a bacterium carried by fleas have been blamed for decimating societies including 14th-century Europe during the Black Death when it wiped out roughly a third of the population, including in Basel. The disease comes in three forms, but the best known is bubonic plague which is marked by buboes, or painfully swollen lymph nodes.