The short answer is sometimes yes, sometimes no, like with a lot of software. It's not universal protection. For example, there if someone mails you an attachm

The short answer is sometimes yes, sometimes no, like with a lot of software. It's not universal protection. For example, there if someone mails you an attachment that you open, there is a program that can find your dropbox and encrypt all your files....including on your local machine (whatever filesystem elements are linked to dropbox) and drop new virus vectors on your dropbox directories. Windows Defender didn't notice when it happened in our office. Of course you just revert your file folders in Dropbox (machines that are also linked to that dropbox that aren't the original vector, which should be quarantined) and lose all the work since the last synchronization, but you aren't dead in the water.