mosquito one down

I just hit one mosquito. luckily still havent bitten me yet. Saw a black shadow flying pass. quickly grab the insecticide. spray a few places near me. But din see it. After I sit infront of computer, it flew to my leg and tried to bite me. I hit using my hand and yes, got it.... why so many mosquito around. within one week, i can hit a few. sometimes 2 in a day.