Thawing Frozen Food
Thawing (often called defrosting in the UK) frozen food seems like a simple enough task, however, as ever, the devil is in the detail.
General Advice
Thaw food in the fridge. This ensures, in most cases, that harmful bacteria do not get the opportunity to multiply and in some cases produce associated toxins. Thawing food in the fridge can take between 12-48 hours depending on it's size and composition. Some particularly large items, such as a christmas turkey may take up to 5 days to fully thaw. For a single 200-400g portion of something like chilli assume it'll take 24hrs, so transfer it from freezer to fridge a day before you cook it.
You can speed up thawing time by freezing home-made food in smaller containers, if possible.
Ensure food is fully thawed before cooking as if it's not, some parts may not cook properly and harmful bacteria may survive the process.
Once it's thawed, consume thawed food within 24hrs.
Fish & Seafood
Fish and seafood should not be thawed (in the fridge or otherwise) in airtight packaging or containers. This is because it may contain clostridium botulinum bacteria type E, which thrive in low oxygen environments. Clostridium botulinum of all types produces botulinum neurotoxin (BoNT), which is the most potent and deadly neurotoxin known to humans. Consuming this neurotoxin leads to botulism, a rare but potentially fatal illness.
Consuming just the endospores of this bacteria may lead to infant botulism for children under the age of one year, which is why it is recommended children under this age do not eat honey. Older humans are not at risk from these endospores. It's interesting the same recommendation is not made for fish and seafood which may also contain spores.
Other types of clostridium botulinum (or it's endospores) are found throughout nature, including in soil which is why you should always wash fruit and vegetables.