Can I get a Virus by downloading maps over Battle.net?
No. Viruses are transmitted through running an executable that has been infected. Maps delivered over Battle.net are data only, and you cannot get a virus from merely viewing data. This is the same reason why you cannot get a virus from simply looking at your email -- you would have to run an attached program, such as the Word document included with the Melissa virus.
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I may be wrong, but I pretty much agree with that statement.