JJ has a post on her blog about the unknown status of the water glasses at hotels.
I drink a lot of water and have a water softener - reverse osmosis system at my home. I tend to notice a lot of the interesting flavors and aromas in water because of that. JJ makes a good point about figuring out where those flavors and smell are coming from, this also applies to security. Once you recognize that you have a problem, you need to figure out what is the source of the issue. How did the infected file make it into your network - e-mail (the water itself), a portable device or some other means (the glass or pitcher). If you know what is causing the issue, you have a better chance of avoiding other problems in the long run.
If your network keeps getting infected but your e-mail is not the source, it is time to start looking at other sources. An infected laptop or thumb drive could be indicative of problems at an employees home, if you don't work to fix that issue you will end up with the infection returning again.