How dangerous is uncooked pizza?

Chris: You should always make sure things are fully-cooked.

The reason for doing that is that bacteria may have settled on the food when it was being prepared. They may have been there only at very low levels when the food was being prepared, but if it hasn't been stored very well, those bacteria can persist and multiply.

Bacteria can easily survive the temperatures in your freezer. Bacteria can survive being frozen down to -200 degrees.

And if you don't warm food sufficiently thoroughly to kill the bacteria, you get some bits of the food which are warm enough to sustain a much better environment for the bacteria so they grow really fast.

When they grow very fast they can either infect you, like salmonella, or they can produce toxins into the food like Staphylococcus aureus. This produces a toxin which can cause diarrhoea and vomiting. That toxin is also heat stable, so even if you thoroughly re-heat the food, unfortunately you can still get problems because the toxin is still in the food.

The moral is to make sure you cook things absolutely thoroughly!