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No bacteria in their normal state are spores, they are single cell prokaryotic organisims which multiply using a process called mitosis (1 cell splitting equally into 2). However under adverse conditions some species can sporulate (form spores, endospores), some thermophiles (heat tolerant) such as spp. Clostridia for example, when faced with high temperatures will turn into spores. When an organism is in this 'vegetative' state it is inactive and requires 'normal' or certainly more beneficial conditions to allow it revert back to its original state.
This is why the organism Bacillus cereus is so dangerous when warming pre-cooked rice. This organism lives on rice(amongst other places) and during the cooking process it is capable of sporulation. Once the rice is cooked and being consumed a cooling process naturally takes place. When the rice cools sufficiently the organism will change back from the vegetative to the active state and start to multiply. To begin with there won't be enough organisms on the rice to give an infective dose. But once you have finished with the rice, you may leave it on the side for couple of hours, then put it in the fridge. In the fridge the temperature might be between 3-8 deg C, warmer in some cases. The next day you decide to have the rest of that rice for a meal, and warm it up in the microwave, except there are a lot more organisms on the rice because they have been having a party all night and the temperature you have reached by warming it up was probably insufficient to kill the bacteria (most of these organisms can only endospore form once and can't keep slipping in and out of the state, so a good heating second time could kill most of them).
Most spore forming bacteria cause food poisoning by producing something called an endotoxin, so are referred to as toxinogenic and it is the ingestion of this poison and not the organism which causes food poisoning (as opposed to 'enteric' bacteria e.g. Salmonella spp. etc. which cause food infection as an organism present in the body)A distinction someone suffering from food poisoning, I suspect, wouldn't care too much about.
Just as a foot note, Bacillus cereus is also the organism which causes 'bitty' cream on the top of your milk when its few days old. So be careful what you drink.
I hope this helps
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