Poorly Apple tree

One of our Apple trees looks poorly. It has about 50% of the foliage it should have, there are no leaves at the end of most branches, and some leaves are yellow/brown. The nodules on the trunk, where previous branches were cut off, don't look healthy to me. We have another apple tree, health and only 2 metres from the poorly one, and I want to make sure that whatever it is, it doesn't pass to that tree. Any advice gratefully received.