At least 260 cases in Herefordshire already this year.
Residents and visitors are being reminded to protect themselves against the Blandford Fly.
The Blandford fly is a 2 to 3mm bloodsucking black fly, which lives near areas of water and gives a particularly nasty and painful bite. The fly normally bites ankles and legs, as it flies low to the ground and is most common during May and June.
Healthcare professionals have advised that bites can be preventable if the correct precautions are taken.
There have also been cases of people who have been bitten booking appointments with their GP. While the bites can be uncomfortable, a visit to your GP is usually unnecessary, and your pharmacist should have all the advice and products you need to reduce discomfort and avoid infection.
Karen Wright, Director of Public Health for Herefordshire, said: "As the weather starts to get warmer, it is likely that we will see more cases of Blandford fly bites across the county.
"We encourage people to take precautions if they're out and about on our rivers and streams, especially over the forthcoming bank holiday weekends, by covering up and using a good quality insect repellent. If you do get bitten, it can often feel very uncomfortable with swelling, blistering, joint pain and sometimes a high temperature. However, there's normally no need to visit a GP.
"If you do feel unwell after a bite, please call NHS 111 or visit your local pharmacy for advice on how to ease your discomfort. For the majority of people, the symptoms will only last a few days before clearing up."
Dr Mike Hearne, Medical Director at Taurus Healthcare Ltd, said: "The Blandford fly tends to cause quite a severe reaction on the skin with prominent swelling and discomfort resulting in significant numbers of people attending their GP surgery or A&E for advice.
"Prevention by covering up is ideal, but if you do get bitten, follow the advice on the NHS choices website about 'bites': elevation of the limb and a cold compress are really important to reduce the swelling. An antihistamine may also help. Often people are concerned about infection, which normally presents with a large red area (often 10cm or more) which appears to spread. This may be associated with a fever or feeling generally unwell.
"If you are concerned about a bite, please phone 111 or your local GP surgery, and only attend A&E if it is clearly an emergency."