A public inquiry is under way to investigate the disaster that befell people with haemophilia. But sometimes when you don't even know the virus exists. A paper written in 2012 said this:
'There are 219 virus species that are known to be able to infect humans. The first of these to be discovered was yellow fever virus in 1901, and three to four new species are still being found every year.'
Really the only way forward is to remember that all treatments carry risks, and the benefit conferred by the treatment must be worth having.
Here is the latest annual report from NHS Blood & Transplant on infections.