I second the eucalyptus oil; also tea tree oil works wonders.
In fact, I used to get bronchitis every other year or so and honestly thought I'd just keep getting it. (I was a light smoker about a decade ago but quit years before this trend in upper respiratory infection ever happened)
Once I discovered herbs and aromatherapy ( wonderful methods of healing and the provenance of people without health insurance!) my tendency towards such illness dropped away dramatically.
I have found that gargling 2-3x per day with warm water to which a couple drops of tea tree oil have been added, at the first twinge of sore throat or tickle, helps keep upper respiratory infections at bay. This includes strep throat, too, but if that is suspected keep an eye on it as it can grow serious if untreated.
I hate taking antibiotics and have found this remedy an excellent preventive measure, along with eating right, getting enough rest and exercise, etc. It also helps to keep the air in your home in winter moist and fresh. Boil a pan of water on the stove and add dried citrus peels, cloves, cinnamon sticks, rosemary, and any number of essential oils (lemon, orange, grapefruit, eucalyptus, lavender, clove are all good): this humidifies and purifies the air.
If you do need an antibiotic, try colloidal silver first. It is a natural antibiotic which does not wreak havoc with the immune system like standard ones do.
Drinking an herbal tea made with rose hips, echinacea, chamomile, lavender and red clover can help fortify immunity and fight the infection, too. Go easy on the echinacea; a little goes a long way! These other herbs help mask the taste of it and this is a very pleasant combination. More rose hips add more vitamin C and a nicely tangy taste. You can add honey to it as well.
(I have all these herbs readily available so if you can't get them where you live PM me and I will make up a little packet for you).
Best of luck. Do quit smoking if you can; you'll love life more, I bet!