Viral pathogens are extremely hard to find. When we test for viruses in the human body we look for the antibodies because to get enough virus material to recognise it is extremely hard. Viruses hide inside our cells and stay there dormant slowly creeping and expanding throughout the body when they aren't in an acute phase of infection. You need tissue samples and then RNA testing to find them, its expensive and extremely invasive and has a high chance of finding nothing unless you are testing the right place.

This has been a big problem with chronic illnesses. Even now despite the US military having great indicative data that EBV causes MS there hasn't been much luck in finding the virus in sufferers bodies, just the antibodies and the usual profile of cytokines associated with the viruses presence. Only recently Dr Bhupesh Prusty found EVB and HHV6 in the autopsy of 3 ME/CFS patients brains and none in controls strongly suggesting a viral cause for the disease but its not a test we could do on a live person and they never found these viruses in the patients fluids when they were alive.

Their theory is "the AZ vaccine causes clotting problems in people who (a) have some particular relatively common gene, and (b) have previously been exposed to a pathogen that is similar from their immune system's point of view to the adeno-virus that was used a vector". Point b means that the pathogen is something these people have been exposed to in the past: the only trace is the immune response that got reactivated, so identifying which particular pathogen it was is not easy.