``But doesn't the fact that people have the source code make it easier to break into the operating system?''
This one is classic FUD that might initially look difficult to classify. It also points out a good method for disguising FUD. This is, of course, an example of FUD#2 (outright fabrication), which the author is hoping to slide by you because he has phrased it as a question rather than as a statement.
Point to the fact that Linux is a Unix look-alike and that there has not been a single incidence of a virus ever afflicting a Unix or Unix-like system. Also point to Eric S. Raymond's classic ``The Cathedral and the Bazaar'' and point out his insight that ``all bugs are shallow with enough eyes on them.'' ( http://www.earthspace.net/~esr/writings/cathedral-bazaar/ ).
Finally, here's a nice quote from Paul Baran, an early security specialist (worked at RAND from 1959 onwards): ``if one cannot safely describe a proposed system in the unclassified literature, then by definition, it is not sufficiently secure to be used with confidence.'' [Vogt 1]