When your must or wort gets contaminated due to lack of sterilization it's unlikely to become poisonous. It's more likely to become vinegar. It's the fact that

When your must or wort gets contaminated due to lack of sterilization it's unlikely to become poisonous. It's more likely to become vinegar. It's the fact that you're using uncontaminated fruit juice or boiled wort that makes the beer or wine safer to drink than water out of the cistern beside the outhouse.