Drive by downloads are some of the worst traps on the internet. You get infected by Legitimate sites, which are fed advertisements by compromised Open Ad servers – As servers feed the legit site an special iFrame that redirects the unsuspecting user to an Exploit Kit (EK) site. The EK runs and does all kinds of system checks to see how it can compromise your computer. Adobe Flash is at the top of the list of ways these EKs get in and own your machine. Adobe Flash has a Long History of Zero day and other vulnerabilities . Specifically, Flash is used by Angler Exploit Kit; and bad things can happen if FLASH is running on your machine. Most major sites have killed FLASH in favor of HTML5. Good practice just to kill FLASH if you have not done so. Protect yourself. Protect your family.
Kill FLASH in Firefox
Kill it in IE ( if you are using IE)
Kill it in Chrome