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There exists many myths on the rand ‘virus’. Some consider it to be an Easter Egg. The truth is, it is just an un-documented MS Word function to insert dummy text
The Microsoft Knowledge Base article How to Insert Sample Text into a Document in Word  explains the use and syntax of the function:
Microsoft Word allows you to quickly insert sample text into a document. To do this, type =rand() in the document where you want the text to appear, and then press ENTER.
=rand(1,1) inserts one one-sentence paragraph of text.
=rand(10) inserts 10 five-sentence paragraphs of text.