There are relatively few laws (though some countries do have them) preventing the secure exchange of viruses between consenting individuals, though it is clearly irresponsible for you simply to make viruses available to anyone who asks. Other requests come from people you have never heard from before. Using strong encryption, you can send them what they have asked for by almost any medium (including across the Internet) without any real risk.
Some requests are easy to deal with: they come from fellow-researchers whom you know well, and whom you trust. If you are active in the anti-virus research field, then you will regularly receive requests for virus samples. (read the complete text, it contains important information) It was decided not to change the file itself for backward-compatibility reasons. The content of this documentation (title-only) was adapted 1 September 2006 to add verification of the activity of anti-malware or anti-spyware products.The definition of the file has been refined by Eddy Willems in cooperation with all vendors.This file used to be named ducklin.htm or ducklin-html.htm or similar based on its original author Paul Ducklin and was made in cooperation with CARO.