It depends. Think of it this way. Say you have ten frequencies: 1-10. Your spread spectrum device will send information on all ten frequencies but another device may send it only on one frequency (e.g. frequency 7).

So say your old, leaky microwave is blasting out power on frequency 7. You totally wipe out your frequency 7 device but only 10% of your spread spectrum device. (i.e. Frequency 1-6 and 8-10 come through without interference). With a little error correction you can fully recover the spread spectrum data.

Of course it's a little more complicated than that but you should get the idea.