What is considered to be the worst disease ever seen? and how many people have been or are affected by it?

The the worst disease ever know to mankind was Black Death also known as The Plauge.

This disease spread all over Europe, even though it spread to other countries, The Plauge was mostly in Europe. It affected/killed many people of Europe's population in the 14th century. the Black Death is estimated to have killed 30–60% of Europe's total population. In total, the plague may have reduced the world population from an estimated 450 million down to 350–375 million in the 14th century.