Have just read a note from someone who spent 4 days in hospital on a drip caused by insect bites that got infected (while mowing).
The bacteria concerned has a 4 to 24 hour development phase. Skin becomes red and inflamed/swollen and sore then you get other sumptoms such as migrane, high temperature and shivering.
First, the doctor has to distinguish whether or not the skin inflammation is due to an infection. When it is difficult to know if the inflammation is caused by an infection, doctors sometimes treat with antibiotics just to be sure.
I always apply a DEET based roll on (from Boots) at one customer's site due to the number of horse/thunder flies that descend on me during hot/muggy days.
Hope this helps,