Sneezing is usually caused by a reflex due to nasal irritation or irritation of the diaphragm.

http://priory.com/vet/vetrept2.htm

"reptiles lack a diaphragm"

http://animal.discovery.com/guides/r...natomy_02.html

The respiratory system of snakes includes the trachea (windpipe), bronchi, lungs, and air sac. The trachea originates at the glottis, at the back of the oral cavity, and terminates near the heart, where it branches into two bronchi. The left bronchus leads to the left lung, which is reduced or completely vestigial. Vestigial organs are small, degenerate, and non-functioning. The right bronchus leads to the right lung, which is elongated. The anterior portion of the lungs is vascular (with blood vessels) and functions in gas exchange, but the second half of the lung is an avascular (without blood vessels) air sac that extends into the tail region. The air sac performs a hydrostatic function in most snakes, regulating pressure inside the body cavity. Because snakes do not have a diaphragm, air enters and leaves the lung due to action of the body muscles and movement of the ribs.

http://nationalzoo.si.edu/publicatio...gguamsnake.cfm

A human can catch the flu without ever actually making physical contact with someone who is sick. While snakes dont have diaphrams making it impossible for them to sneeze on each other a healthy snake likewise can contract a paramyxovirus via secondary contact, such as slithering through a sick snakes secretions.