When cold weather comes, often it's followed by the flu, heavy colds, & for those who suffer from asthma, it often gets worse in damp conditions. A few changes to your yoga routine can either prevent or reduce these winter malaise-s.
I had a break from my ongoing yoga practice for quite a few months: I had a damaged foot & the asanas were making it worse, & I was so disheartened by the bad behaviour of so many of yoga's famous teachers & gurus, that I just didn't want to practice.
But all things change, & this feeling changed too. For when you practice yoga consistently, the body always craves to return to it, because it makes us feel so good. Yoga, of course, doesn't have the monopoly on feeling good...walkers, runners, skiers, surfers, all sorts of people practicing their physical activity of choice will tell you how good their activity is, how much they miss it if they can't do it.
And these consistent physical activities keep our immune system working well, we get less sick. The asanas (postures) of yoga really were all about the glands, the organs, the nervous systems, stimulating the chakras, but since B.K.S. Iyengar introduced the asanas with "correct" postural alignment & also "correct" muscular control, the established "norm" of yoga asana practise changed. I teach it this way too now, & have done so for over 20 years, however for my own practice, I am just about actually doing it, otherwise it's just too overwhelming.
Yoga pranayamas are another way to keep our immune systems working well, to minimise or prevent (as the case may be) sickness. When we do the cleansing pranayamas like bhastrikas, kapalabhati, breath of fire, we build up a protective layer of negative ions around the nose. Negative ions are molecules in the air, & although we usually associate the word "positive" with being the "good" aspect of something, in this aspect the negative ions are the "good" ions/molecules: they make us feel good, they are good for our health. And when we have a strong auric supply in a cachement around our nostrils, as you have when regularly practising these cleansing pranayamas, we are then inhaling these negative ions, thereby maintaining good health.
Generally speaking, unless you have high blood pressure (in which case, don't do it!), bhastrika is a wonderful winter pranayama, it warms the body, & very importantly, it stimulates our own personal supply of prana/life-force, which is often in short supply in winter, as much of our prana often comes from the sun. And it builds up the negative ion cachement mentioned above.