Will spoiled food always make you sick?
What most people don't get when it comes to food safety: Spoiled food has a chance of making you sick. 

When food is visibly spoiled, it has large bacterial colonies growing in it. This means that it has been exposed to conditions which were promoting bacterial growth. Anything which was present on your food will have grown, unless outcompeted by something else. 

The bacteria which make food gross are rarely the ones which make you sick. But if they had the chance to grow, the real pathogens had a chance to grow too. If they were present, then they grew, and you will consume them, together with whatever waste products their colony produced. 

Assuming that there were infectious bacteria on your food, you will ingest them. From there on, it depends on a ton of factors whether you will get sick or not - the amount you ate, the acidity of your stomach contents, the state of your immune system. The last does not mean "you'll only get sick if you're immunocompromised" - it is like being around a person with the flu, even if your immune system is intact, you can still catch it. 

There are pathogens which cannot infect you, but simply produce toxins. Botulinum is the poster child for these, but I think B. cereus also works through a toxin. In this case, eating the food poisons you. Depending on the type and amount of toxin ingested, as well as the current state of your body again (mostly liver function), the symptoms can run the entire spectrum from "so weak you don't notice them" to "you die despite being correctly diagnosed and treated". 

Then there are a few more exotic possibilities, for example Hep C cannot cause liver cancer alone, but it does so (sometimes) in the presence of a type of toxin produced by moulds. But this is not actually considered foodborne illness any more, even though it's possible that eating mouldy food was the trigger. It falls outside of the scope of food safety, as the fault cannot be pinned on a specific dish you ate. Nevertheless, it's one more reason to not eat spoiled food. 

There is also the possibility that your spoiled food has no pathogens at all, and the spoilage microbes outcompeted all the bad guys, and then of course nothing happens. 

So, to sum it up: if you eat spoiled food, there are four possible outcomes: 


You get sick (right away or with an incubation period of 2-3 days) and notice it 
You get sick (right away or with an incubation period of 2-3 days) but the symptoms are so weak you don't even notice it 
You get a different health problem, soon after eating or even years later, without knowing that it was the spoiled food which caused it 
Nothing happens. 


There is no way to predict which one will occur. Actually, these outcomes are possible with any food you eat, including safe food, but eating unsafe food increases the probability of the first outcomes a lot, and eating spoiled food increases it even more.