Virus infected boot camp
Most viruses (like 99.999% of them) are unable to transfer between operating systems for many different reasons, but theoretically a virus could spread between the two operating systems. What is more likely is that the virus would corrupt the partition table itself.

Take a look at the following links for more info:


Quora: https://www.quora.com/Can-a-Windows-virus-infect-Mac-OS-X
MacRumors: https://forums.macrumors.com/threads/can-windows-viruses-acquired-in-os-x-infect-the-boot-camp-partition.1075192/


Please note my percent is not based off of any legitimate statistics, it is exaggerated.