It depends on the area. In some places, mosquitoes were very active all the time, and spread deadly diseases like malaria. In other areas, there were no mosquitoes, or almost no mosquitoes - or the ones that were present were only active at times of the day when humans were less active.

So, it depends.

Also, in the US, many of the species of mosquitoes we have now are not endemic to the country. There are mosquitoes that originated in Asia that are now here, and in my area, we now have mosquitoes 24/7, instead of only at dawn and dusk (as was the case when I was a kid.) So just because there are lots of mosquitoes where you live, that doesn't mean that that same level of mosquitoes was present in all parts of the ancient and medieval world (even if they had similar climates to the US midwest.)

Edit: added pluralization