Why asthma is both over diagnosed and undertreated. Professor Mike Thomas and GP Dr Margaret McCartney discuss this apparent contradiction and look behind recent headlines that half a million children in the UK could be taking asthma medicines they don't need. A new study finds that putting doctors under pressure or being a difficult patient may backfire, inducing them to make diagnostic errors. With scarlet fever and measles in the news, Margaret McCartney gives a quick guide on the key symptoms as both diseases have a characteristic rash. A listener has emailed to ask about visual snow, a condition where your vision is like an untuned TV set. World expert, Professor Peter Goadsby explains the latest understanding of visual snow, and says that even 15 years ago it hadn't been universally accepted as a condition. Plus the first in the latest Inside Language series with Margaret and Dr Carl Heneghan of Oxford University. This week, they discuss confounding factors and why they matter to your health.