I am asthmatic and have bad allergies and respiratory problems. In the States, all that I was really affected by was allergies (damn you, Spring!) and occasional respiratory illness like strep throat or bronchitis. I never used my inhaler, and they usually expired without being used. But since I've moved to London 6 years ago, I've had to go on a preventative steroid inhaler (that I have to use twice a day or I stop being able to breathe properly) as well as my normal inhaler. My allergies have also worsened and I get respiratory illnesses at least once or twice a year, along with other general illnesses. A lot of it (for London at least) is due to the terrible air quality. But I also think that being in very densely populated areas increases the probability of getting ill. I always wear gloves during the winter when I'm out, and try to clean my hands frequently.
Also, I make sure that my house is mould free. My asthma went out of control years ago because there was black mould in my closet due to damp walls. In old houses, dampness is a problem and it's best to check once and a while/heat and dehumidify the house to prevent sickness, especially with this weather.