On Friday, 34 patients were told they could have contracted the potentially deadly virus following a visit to Edinburgh Royal Infirmary in July.
One of those affected, 52-year-old Liz Macnab, spoke out, asking why it took so long for NHS chiefs to identify the issue.
The tax consultant, from Edinburgh, spent several hours at the hospital's accident and emergency department where she received a dose of intravenous antibiotics for a mystery illness. But she ended up spending weeks in a ward after reacting badly to the medication and collapsing.
She is now recovering at home but said the arrival of the alarming letter from NHS Lothian on Friday was the "worst possible news".
Ms Macnab said: "It has taken six months to let me know that I may have contracted this virus because of a contamination issue and I am not the only one. How many people pass through A&E departments every day? Dozens if not hundreds."
A spokesman for NHS Lothian said they had opened a special helpline. He added: "We took action as soon as we became aware of it and sent out those letters and followed them up with phone calls."