
The FDA has certified a portable air filter called the Plane Clean (read about it at Gizmodo ) , which mounts to the small air vent above your seat and removes about 99.5% of allergens, bacteria, and viruses.
The filter is more effective than oral immune-system boosters and less conspicuous than masks, and is also inexpensive – the units are available at Target and Amazon.com for about $20. I wrote about it here.