A "coding error" on the NHS website exposed users to harmful websites rather than health advice.
More than 800 pages on NHS.uk automatically redirected unsuspecting users to pages that contain either malware or advertising.
The affected pages were highlighted by a user who posted details of the problem on Reddit.
In a statement, the NHS said its site had not been maliciously attacked and that it had fixed the problem.
"An internal coding error has caused an incorrect redirect on some pages on NHS Choices since Sunday evening," a statement explained.
"Routine security checks alerted us to this problem on Monday morning at which point we identified the problem and corrected the code."
Typo blamed
Reddit user Muzzers said he had stumbled across an infected page while he had been browsing for information about the flu.
"Digging a bit deeper I found hundreds more pages which redirect to either an advertisement or malware infested page," he wrote.
Users trying to find details on dementia, pregnancy, vaccinations, mental health and other areas also found themselves sent to the malicious pages.
The fault occurred due to a typo within the NHS website's source code.
A developer accidentally wrote "googleaspis.com" rather than "googleapis.com" when creating the site.
The mistake went unnoticed until the incorrectly-spelt address was registered by someone in the Czech Republic over the weekend, and was then used to capitalise on the error.
The NHS said the site would not be completely clear of the problem until later on Monday.
It added: "NHS Choices is treating this issue with urgency and once resolved we plan to undertake a thorough and detailed analysis to ensure that a full code review is undertaken and steps put in place to ensure no reoccurrence."
Oops.
