Web/iOS: You can never be too careful when it comes to contagious diseases. Sickweather feeds the hypochondriac in you by sending alerts when you are near a place where someone has reported an illness.
The iOS app tracks your location data to figure out your position. It also scans social networks to see who has reported symptoms of an illness. (The makers claim their algorithm is smart enough to distinguish between "I'm feeling sick" and "That's a sick beat!")
You can set up Sickweather to look out for reports on different types of illnesses, which are then displayed on a map of your area. It also has four broad "grouped illnesses" maps: respiratory, gastrointestinal, environmental and childhood. If you see a cluster of sick reports around a particular area, you probably want to avoid grabbing any takeaway in the vicinity.
If you aren't using an iOS device, you can still use the Sickweather web app to check your location or surroundings for illness reports. Hook it up with your Facebook and Twitter and you will see friends who are suffering, letting you send some words of encouragement or run as far away as possible.