The ransomware got into the systems via infected emails and other internet links. Those affected will have a message on the screen telling them that their files have been encrypted and will only be released upon payment of 300$ worth of bitcoins.
It seems that Microsoft patched the issue earlier this year, but only on version of the Windows operating system that it continues to support. However, up to 90 per cent of NHS computers still run Windows XP which was released in 2001. Microsoft cut support for it in 2014.
Windows XP was very popular because it was robust. It followed in the wake of Windows ME (Millennium Edition) and Vista both of which were bloated and buggy.
Since then Microsoft introduced Windows 7 which stripped out all the bad parts of Vista, the equally unpopular Windows 8 and the latest -Windows 10.
The issue for the NHS is more than just updating the operating system on its computers. Many of the machines in use are simply not powerful enough to run later versions. An upgrade would cost more than the cash strapped service could afford.