Is running really bad for knees?

I've been running nearly every day to loose weight, for about 30 to 40 minutes non stop. Even though I'm still overweight I can run 4 km non-stop. People tell me how to much excercise is bad for your knees, especially running, but where I live it's pretty much the only option I have right now. I want to get an excercise bike soon, so that I can use that aswell. But I've never noticed a problem with my knees from running, if anything I think my knees are better. Once I'm able to get to the gym again regulary I'll be able to do swimming again and use the other excercise equipment there, but until then I enjoy running. It just seems these days that no matter what you do, there's some so called 'professional' that says that it could kill you. I saw some article online citing people who died of heart attacks during marathons and runners who developed cancer... not that I believe that, as the cases they mentioned were likely to have been exceptional incidents.