Fly strike kills thousands of rabbits. Flies lay their eggs on the rabbit's dirty bottom. The eggs hatch into maggots which eat their way into the poor rabbit's flesh. The rabbit dies from being eaten alive - a slow horribly painful death.
Fly strike is a veterinary emergency. A rabbit can die rapidly so get it to the vet as soon as possible.
You can protect your rabbit by cleaning the hutch every single day in warm weather and carefully inspecting its bottom twice daily in hot weather. If there are moist night droppings in the hutch, then see the earlier FAQ about dirty bottoms and adjust its diet.
Buy Rearguard, a special spray for its bottom, which will protect it from fly maggots for several weeks. You get this from your vet and follow instructions. Another possibility is Xenex Ultra, a spot on containing permethrin, which kills or repels adult blowflies before they lay their eggs. (This is toxic to cats)
Still clean the hutch daily and inspect the bottom twice daily in summer as well. Be particularly careful with long haired rabbits. Insect repellants and fly screens sold over the counter are no substitute for Rearguard or Xenex Ultra.
Get advice from your vet because it is so easy to get it wrong with possible fatal results. If your rabbit has mites and fleas, it may be run down or sick so ask your vet to check for tooth problems and illness, rather than just asking for a flea treatment. Choosing which flea or mite treatment is tricky. Some poison rabbits.
NEVER use an over-the-counter flea treatments sold for dogs or cats. Many of these kill rabbits. Even prescription treatments for dogs and cats can also kill. Remind your vet of this when you are asking him for an anti-flea treatment. Ask your vet for Advantage (Imidaclopid), which is safe for rabbits.
Fleas can pass myxomatosis from one rabbit to another so zap them fast, as well as vaccinating against this fatal disease. As well as treating the rabbit, you may need to treat the environment (hutch, run or your house) so ask your vet for a spray, which is safe for rabbits. Follow the instructions EXACTLY. Treat the whole house if the rabbit is a house rabbit. Treat the whole hutch and run if it is an outdoor rabbit.
Stronghold (selamectin) from your vet can treat mites. In the UK the spot-on Xenex Ultra (permethrin) (see fly strike FAQ above) will also control fleas and Cheyletiella mites. But it is poisonous to cats which might be a concern in a household which includes rabbits and cats. Another UK spot-on, Xeno 450 (ivermectin) kills pests under the skin surface like the sarcoptic mange mites, ear mites etc. It will also kill Cheyletiella and ticks. Vets get irritated by clients who ask for drugs by name so use some tact here!