Bacon is pork of course, but, to comply with food regulations, both are treated with nitrites (and nitrates) to preserve the meat and prevent bacteria from forming. These chemicals break the Paleo ethos, not the meat itself. It is illegal in many countries to sell ‘raw’ meat (or milk for that matter), hence the elimination from a paleo diet on practicality grounds.
Food preservation initially relied on salt, fermentation, drying and canning. Before the age of refrigeration, these methods were used to keep foods from spoiling. Research about nitrates began in the 1920s, where it was found to effectively kill many strains of bacteria that other preservation methods missed.
Sodium nitrate and sodium nitrite are food additives commonly used in cured meat. Its main function is to inhibit clostridium botulinum bacteria from producing the toxin that causes botulism. Botulism is a life-threatening illness that results in paralysis and eventually death. Nitrate combined with salt is extremely effective at inhibiting the growth of clostridium botulinum. Sodium nitrate also contributes to the flavour and pink colour of cured meats.