Yoga in the classical sense is an eight-limbed path set out by Patanjali, of which posture is only one limb. The other seven cover ethics, self-discipline, breath, withdrawal of the senses, concentration, meditation and absorption. The physical practice most people mean by "yoga" is asana.
Regular practice measurably improves flexibility, balance and strength, and there is good evidence for benefit in chronic low back pain, in balance and fall-prevention among older people, and in anxiety and depression as an adjunct. Modest reductions in blood pressure have been shown. It also, less measurably, teaches attention.
Learn from a teacher in a room, at least at first. A book or a video cannot see that your knee is tracking wrongly. Practise on an empty stomach, ideally in the morning. Work to the edge of comfort, never into pain. Consistency beats intensity: twenty minutes daily is worth more than two hours on Sunday.
Tell your teacher about any injury, high blood pressure, glaucoma, hernia, recent surgery, or if you are pregnant. Headstand and shoulderstand are not suitable for everyone and cause a good proportion of yoga injuries. Strong pranayama should be learned in person, not from a screen.
This site is a personal account and a record of traditional practice, shared for information. It is not medical advice, not a diagnosis, and not a treatment for any disease. I am not a doctor.
If you are unwell — and especially if you have a serious illness such as cancer, diabetes or heart disease — please stay under the care of a qualified physician. Nothing here should replace or delay that care.