Well BBC places smallpox around 300 million people in the 20th century alone while wikipedia places it around 300-500 million (I placed the links below). Polio death rates don't even come close, the highest estimates for the Bubonic Plague are around 200 million, the known flu pandemics have killed around 50-60 million people (likely more but we don't have enough information, but most likely not high enough to surpass smallpox). From all of our current information, Smallpox is the deadliest of those diseases.
Good quiz, but can't go along with the 'Moons of Jupiter' question, I believe there are at present 67 known moons of Jupiter, I went for 64 as the nearest. Finding new moons all the time.
http://www.bbc.co.uk/history/british/empire_seapower/smallpox_01.shtml
https://en.wikipedia.org/wiki/History_of_smallpox