I believe you might mean malware files in general and not only viruses. The media often puts them on one level although viruses are a small subcategory of malware that show the behaviour that quietman7 describes.
For instance, if you need to send (or have already sent) to third parties an infected file for scanning, how do you know it does not contain any of your personal data?
People who send in files usually do that, because they don't know what these files are and what they contain. In that case you can't know if the file contains personal data.
Some malware will store information in their own log files like credentials they stole. If you happen to send one of those files, because they are unfamiliar to you, you also give away personal data. Only send files to third parties that you trust.
Edited by Curie, 27 March 2016 - 07:35 AM.