Why are disgusting-tasting foods usually the most nutricious?

Wasn't nature supposed to have built us for survival? When you feel cold, your body shivers to keep you warm. When your skin gets cut, platelets clot the blood around the area. When you get a disease, your immune system kicks in. So why does the tongue want to reject nutricious food? Doesn't the brain know that the food can make our immune system stronger?

If we think along the lines of Darwin theory, it would be suggesting that the world will slowly be rid of weak-minded people-people who cannot bare to swallow 'bluaarrrgghh!!!'-tasting foods. Hmmm....