Announcements by security firms indicate that the above listed companies are exposing your systems to unnecessary risk of malware infections to boost the bottom line. (Links below)
Case in point: My daughter, while attempting to watch a youtube video, was redirected to a malicious flash player download. This download installed a rootkit on her system that played audio ads in the background continuously. While not harmful to the system, they rendered it pretty much useless for any audio applications due to the audio ads constantly playing in the background. The only fix was a total system wipe and OS reload.
Is there a solution to this problem? Short answer is no. While many commercial antivirus applications (AV) do their best to block these, there is not a 100% fix for prevention. This is due to the fact that the code writers for these things far outnumber the ones trying to stop it. As fast as the AV companies deploy a fix the bad guys change the code in a never ending battle.
What can you do? For starters, keep your AV up to date. Many charge for this but Microsoft also provides a free one (Security Essentials/ Window Defender) that is much less intrusive and less demanding on your system. You may go the paid route but keep in mind that just because you pay for it does not make it any better than the free one and in many cases those may drag performance down in the name of protection.
The Yahoo, Youtube, Amazon story: http://www.theregister.co.uk/2014/09/10/big_names_caught_in_kyle_and_stan_malicious_ad_attack/