Greek missionaries took their alphabet (Cyrillic) to the illiterate Swedes/Rus.
Sorry to butt in, but that is decidedly incorrect. Slavs have had an original alphabet, Glagolitic, before St. Cyril and Methodius set out on their evangelical mission — as documented by the very same gents (see http://en.wikipedia.org/wiki/Glagolitic_alphabet#Origins_of_the_Glagolitic_characters
That being said, there are some surprising and rather amusing similarities in structure and word roots between modern Slavic languages and their great-great-great-ancestor, Sanskrit. Numbers sound very similar, some odd edge cases overlap, and even the Sanskrit word denoting sexual intercourse would be immediately recognized by any Russian, Ukrainian, Slovak, Czech, Serb, Croat or what have you.
That might in part explain why this word has maintained its #1 position on every Slavic word pop chart, ever.