Why do we have this horrible smell in our bathroom?

I keep noticing a horrible smell in our bathroom, it seems to be coming from the cupboard where our gas boiler is housed and of course the water pipes. It doesn't smell like gas particularly but more like rotten eggs or stagnant water. It doesn't smell all the time. I know the smell is very similar to that when you have a dead rodent in the wall. Could that be the problem or is it a problem with our pipes?