In general this line of argument is questioning how we can know that a particular virus causes a particular disease. It ignores decades of technological improvements developed toward actual viral bulkup, purification, and infection.
Is it easy to know that a particular virus causes a particular disease? Not always, particularly if the disease syndrome is complex. Is it possible to link particular viruses to particular diseases? Definitely. For the more straightforward diseases it's even relatively straightforward to show that a particular virus causes that particular disease, with enough work.
Is it easy to know that a particular virus causes a particular disease? Not always, particularly if the disease syndrome is complex. Is it possible to link particular viruses to particular diseases? Definitely. For the more straightforward diseases it's even relatively straightforward to show that a particular virus causes that particular disease, with enough work.