Summer colds, like their winter cousins, are caused by a large family of viruses. Summer travel puts us in contact with many of these viruses. Colds are spread in crowded indoor areas and on contaminated surfaces, such as door handles, stair railings, etc. The viruses can live for hours on such surfaces, and when we touch contaminated items we can pick up the viruses and transmit them to our nose or eye tissues. We can decrease our risk with frequent hand washing.
Although summer colds occur less frequently than winter ones, there are some special factors that increase the risk of infection: air travel, where we are contained in a small space with hundreds of other people; air conditioning, which removes moisture from the air and causes a drying of protective mucous membranes; and hay fever, which also can disrupt the integrity of the mucous membranes.
If you catch a summer cold, rest, drink plenty of liquids (especially water), use a humidifier in the room where you sleep, and expect to get well in about 7 to 10 days. If symptoms persist after that, see your doctor.
— Source: University of Nevada Reno