We are beacons that are constantly attrating the mosquitoes. Mosquitoes, like ourselves, have senses that lead them to food and in this case the food is our blood. The main senses they use are SIGHT, SMELL and SOUND. We attract mosquitoes in many ways, but you can combat these attractions.
Mosquitoes are attracted to a sweating, breathing human becuase they sense the carbon dioxide given off by exhilation and the chemicals present in sweat. So if you are playing football outside you will probably attract alot of mosquitoes. They are also attracted to scents pleasent to ourselves... mainly perfumes and colonges.
To repel mosquitoes you can use smell repellents such as sprays containing DEET, and products containing citronella... both chemicals are not pleasant to mosquitoes. Another product that came on the market a few years ago the mosquito magnet (http://www.mosquitomagnet.com/)It produces CO2 by burning propane and uses a small fan to suck up mosquitoes attracted by the CO2.
Mosquitoes do not have great eye sight but they can distinguish colour contrasts. If you are wearing a light coloured shirt in the middle of a field you are going to be noticed and attacked. There have been claims that mosquitoes are attracted to a certain colour more than others but these claims have not been proven.
To repel mosquitoes through sight you must wear clothing similar in colour to your surroundings. You will blend in and be attacked much less.
Mosquitoes are attracted to something making noise. A deer running through the normally quiet forest will draw mosquitoes, as will a human running in a field.
To combat the sound you are making you can purchase a device that emits a high pitch sound that humans cannot hear but mosquitoes find unpleasent.
If you are playing football in a field, wearing a white jersey and you are not using any repellent you are going to be eaten alive.