Yahoo data breach in August 2013 much bigger than thought
Yahoo’s owners Verizon have now admitted that every single Yahoo account at the time was exposed during the data breach. That’s roughly 3 billion accounts compromised!
My advice (and I strongly suggest you take it) is to switch to a different provider. Yahoo had a duty of care and they failed massively. If you actually want to keep your Yahoo account (I think you must be crazy), then for heavens sake do everything you can to make sure it is secure. Have you got a strong enough password? Have you enabled 2 step verification? You know the drill.
If you want to know how to enable 2 step verification for your Yahoo account, go here: https://help.yahoo.com/kb/SLN5013.html