A massive bitcoin mining virus wouldn't help. Let's say there are 2 billion PCs in the world, as this random site estimates[1]. Let's say each of them has a high-end graphics card.. say, a GTX 1060 that can do 20 MH/s. Let's say your virus can break into every single computer in the world, and run all those graphics cards at 100% without being noticed.
Now your mining power is: 20 MH/s * 2 billion = 40000 TH/s, or 0.1% of Bitcoin's hash power.