Homeowners are being warned to flyproof their kitchens
The number of flies looks set to increase as the heatwave weather continues.
Pest control company Rentokil says it has seen a 74% increase in call outs for fly infestations.
It is warning homeowners and restaurants to flyproof their kitchens and picnics.
"Dining out as well as barbeques and picnics are all prime locations for disease-spreading flies to gather", it says.
"Flies bring with them the increased risk of food contamination, which can lead to upset stomach and a variety of nasty illness."
It says the recent warm weather is contributing to rapid growth in fly population numbers.
A common cause of diarrhoea is from a family of bacteria known as Campylobacter, which can be transmitted by flies.
Dr Colm Moore, area technical manager with Rentokil, said: "The growing popularity of eating out and travelling during summer months, combined with poor hygiene, can contribute to the increased incidents with these pests.
"Flies are one of the most diverse insect orders, with more than 120,000 species of flies worldwide.
"Knowing about the size, habits, seasonality and lifecycle of different fly species, can help to identify the most effective prevention and fly control methods.
"If a small fly problem is left uncontrolled, it has the potential to turn into a serious infestation."
Fly larvae take seven to 10 days to mature into adults, and in their lifetime they may produce 1,000 eggs.
It recommends taking the following steps: