This method has been tested and worked in the UK, I cannot speak for other regions but it should work regardless:
Go to the main KFC website, scroll down into the "contact us" section and go on "email us"
Choose the type of enquiry about a recent visit
After that fill out all the details it asks
Choose a KFC restaurant somewhere in your city
In the "describe your experience" section go on the KFC menu and write down a bunch of food that you "bought" as well as the prices of the whole meal. (generally aim for 20-30, I have not tested anything higher)
Then in the same text box, say that you recently visited this KFC and the food tasted weird/raw/cold/different and after the meal (or the next day after) you had stomach troubles. You can say you had diarrhoeaor a very bad stomach ache, but don't go to the extremes and say you had food poisoning and went to the hospital or something, that is unnecessary.
Once you've described how you had a stomach ache just send the email and you will soon receive a voucher for the price of your meal.
Along with the voucher you might also get a form asking to elaborate on the experience, but just ignore it and keep the voucher.
Enjoy your next big KFC meal for free using the voucher.
OPTIONAL: If you've been to a KFC recently and still have the receipt, that's even better because you can include a picture of it in the email. It will work without a receipt but it's just a bonus.