I've had food poisoning twice in my life and both times it was from food prepared by others. One case was lumpia made by a family in our church and sold at a Bazaar. It was apparently frozen, thawed and refrozen without our knowledge. I was only sick one day. My sisters and parents were sick for three days. The second case was from a restaurant and I was sick for several days that time. I pretty much never get sick and the last time I had anything remotely resembling gastrointestinal was the restaurant meal and that was about 6 years ago. My DH on the other hand grew up with a mother who thought you could leave chicken sitting out on the counter for hours and he had multiple intestinal bugs while living with his parents. Her food handling was not very good. I actually watched her wipe something off the floor with a dishrag and then proceed to use it on the counter and dishes. YUCK. Since we've been married (35 years), he's not been sick with anything but a cold.
The problem is usually tracing back to what meal actually made someone sick. The reason I know it was the restaurant meal is that DH and I had eaten every meal at home together for several days before eating out and he didn't get sick. Food poisoning can take up to 3 days or so to appear.
Point is, frozen chicken cooked in a crock pot usually hits a warm enough temperature to keep it safe in a couple of hours at most. Crockpots sold in the US really aren't slow cookers as they have to comply with FDA regulations and cook at a higher temp than in other countries. If you notice, food in crockpots will boil. True slow cookers do not get hot enough to boil anything.