When we cook food, we expose it to high temperatures, which kills bacteria in it and makes it safe to eat. Raw food, being uncooked, has the risk of containing harmful bacteria like Salmonella, Listeria, and E.coli. If you had cooked it, you would have killed these buggers and your risk would be almost zero. But when served raw, you always risk a chance of that piece of meat containing some of these harmful critters.
The USA Food and Drug Administation Centre for Veterinary Medicine conducted a study and found that of 196 raw pet food samples analysed, Salmonella was found in 15 samples while Listeria was found in 32. There was another study of 35 commercial frozen raw meat-based diets that found E.coli in 80%, Listeria in 54%, and Salmonella in 20% of products.