How to prevent cold in summer hot days?

Preventing a cold is the same in summer or winter.

Your first line of defense is to kill/minimize the viruses that will come in contact with your body.

Viruses don't hold up so well to soap and water - so wash up! When you go to the toilet, wash your hands with warm to hot water and soap, every single time. When you're about to handle food, whether it's to cook or to eat, wash your hands with warm to hot water and soap, every single time. Try to limit contact with surfaces which a lot of other people touch, such as doorknobs. So in a public bathroom, keep your paper towel and use it as a small barrier between your hand and the door before throwing out the paper towel. Or if it's a door that can be pushed open, just use your shoulder.

Wipe down other surfaces with antibacterial wipes or, you guessed it, warm to hot water and soap (obviously, don't get water into your keyboard, but you should be able to clean a lot of your computer).

If it's socially appropriate (I realize it's not always), wash your hands after shaking hands with someone, too.

Your second line of defense is to fight from the side of your immune system. This means eating right, exercising, drinking water, limiting alcohol, and getting enough sleep. Keep a light jacket at your desk for days when it gets unexpectedly chilly or if your transportation has the air conditioning on too high. Keep an umbrella with you in the event of rain, too.

Your third line of defense is to take care of yourself if you feel a cold is coming on. Use zinc lozenges (they are proven to reduce the time and severity of colds, but they don't stop them) at the first sign of a cold. Also, curtail your activities and get more rest than you think you'll need. Give your body a chance to fight off the cold.

I can't say this is perfect -- nothing is -- but it should help. Oh, and talk to your doctor about allergies. That might be some of your summer colds right there.