White Foam in Leaves

It's called "Cuckoo Spit" and surrounds the nymphs of the Froghopper.

It can be left as it has little detrimental effect on the plant.

http://www.rhs.org.uk/advice/profile?pid=490

My wife didn't believe me when I said a little bug lives in cuckoo spit, so I gently spread a little out one day to reveal a little froghopper (and then covered it back up again).

There does seem to be a lot more about this year. I was just about to start a new thread, but saw this one. I noticed that wherever there was some on my gooseberries, the new growth was distorted, so I've been removing it from them. I was surprised to find that in some of the globs there were as many as 4 or 5 frog hoppers residing, which is something I've not seen before. I'm not convinced that they are not causing significant harm to the bushes. Apart from the numbers this year, is there not a chance that they can transmit viruses?