'Shell shock' v's 'post-traumatic stress disorder'

Why is it that shell shock was once a major problem for soldiers and yet is never seen today?
What has this got to do with post-traumatic stress disorder – a prominent problem in modern warfare and yet unheard of 100 years ago?
Professor Simon Wessely of the Institute of Psychiatry, King’s College London, explains the essential difference between the two and what can be derived from it.

This is a part of the lecture ‘Shell shock or Cowardice: The case of Harry Farr’
The full lecture is available (like all Gresham lectures) in its complete form from the Gresham College website, in video, audio or text formats:

Gresham College has been giving free public lectures since 1597. This tradition continues today with all of our five or so public lectures a week being made available for free download from our website.

Source: Youtube