I would have to go digging, but I have seen reports of virus's targeted at Linux et al - but as you say MS is the most targeted OS so there is more in the popular and technical press about them and very little about other OS's.You know different?? Possibly because they are not targeted?? I was informed by a IMAC user that viruses do not get into file systems.. Malware / trackers are not proper viruses..
Here is a link to one report https://www.kaspersky.com.au/resource-center/threats/linux-bash-virus-threat