LONDON‚ April 18 (Reuters) - The World Health Organization and around 500 experts have agreed for the first time what it means for a disease to spread through the air‚ in a bid to avoid the confusion early in the COVID-19 pandemic that some scientists have said cost lives. The Geneva-based U.N. health agency released a technical document on the topic on Thursday. It said it was the first step towards working out how to better prevent this kind of transmission‚ both for existing diseases like...