At certain times of the year there exists the so called flu season that many seem to suffer from due to shift in weather. This is simply unavoidable at times and the chances of sitting next to someone suffering from a cold or flu will be a high possibility. Asking to move to another seat may be an option but the best chance to avoid contraction of illness is to simply uphold hygiene through regular washing of hands and avoiding cross contamination to other parts of the body.
Keep those hands away from eyes, mouths and other entry areas. I tend to head for a shower as soon as I land to cleanse my body, not only to wash away the breeding germs but to freshen up also. Travellers will feel much better after taking a shower post flight. With enough luck, the rest of the days ahead will be faced at full health rather than deterioration due to invading illness.