Cheese is made from milk and uses special fungi to make it hard and give it certain flavours.
For example, if you have ever seen blue cheese, the blue part comes from the fungi that grows in it. If you ever smell it, its very strong and not that nice – I really hate it 😛 . But it gives the cheese a certain flavour that people like. So the strong smell comes from the fungi that is growing in it.
So cheese can never really go off, as it already has fungi growing in it. Most likely what happens is that other bacteria and things can grow and this can make it smell worse.