DropBox is fine ... it doesn't mean you should trust every file shared though. Anyone who get's a virus is unlikely to know how or where they got it. I'm sure they would rather blame something like DB instead of a porn site or pirate music ...
My only problem with DropBox is the crafty pricing ... $100 for 100Gb but of course the only time you actually get to use all 100Gb is just before you have to start paying for 200Gb. Using Google cloud storage can be as cheap as $0.063 per Gb month or $75 for 100Gb per year but you only pay for exactly what you use so it will work out to less than half of DB. Use WebP for images and you double storage or half the price again vs JPEG. That said, it is less convenient for regular users and DB has nice synching apps for most platforms.
They don't distribute malware as part of the platform though.