Microsoft was forced to issue a rare emergency patch Wednesday for its Internet Explorer browser.

The software giant announced the move late Tuesday after numerous security experts advised people to switch to rival browsers until Microsoft fixed the problem.

The recently discovered flaw is considered critical. It allows attackers to take control of computers by steering them to infected websites without the users having to download anything.

So far, more than 2 million computers are believed to have been infected, PC Magazine said.

Microsoft usually releases patches on a set schedule, and this is only the third emergency patch in the past three years.