Note that you don't need to worry about this as the Dropbox desktop client uses the ".dropbox.cache" folder as a staging area to download your files in small chunks and sometimes antivirus programs will pick up these partial files as false positives and continuously quarantine them.
The problem is that Dropbox keeps downloading the file in an attempt to sync the file down to your device. I would recommend adding the Dropbox cache folder to an ignore list so that Dropbox can correctly sync that file and/or temporarily disabling your antivirus. Once synced, you should no longer see these warnings or quarantine messages.
I hope this information helps and please let me know if you have any questions.