This actually sounds more like a memory controller problem, or a bad card. If it's a memory controller issue, then that most likely is due to it being a cheap card. Many cheap cards are sold as low capacity, but are really failed higher capacity cards. There are some low cap cards that can hold significantly more data than advertised, if you "hack" the memory controller. With that said, the memory controller could be allocating data to bad sectors.
Then again, it could just be a failing card, or possibly another explanation. I'm willing to bet it's most likely one of these two problems, if you've singled out the card as the actual problem.
It does sound like you're using the card with a phone, though, so the phone could play a role in this. If so, this would be the wrong section to troubleshoot it. I doubt it's a virus. If it was, it's most likely a virus that's been planted on the memory controller, which means you'd have to reflash it in order to clean it (note that I didn't say reformat, wipe, or anything like that). Viruses on memory cards are most likely going to be infected files, which you would pick up with a typical AV/AM software. The ones that bypass security tools are usually going to be relatively harmless, though.