All the things we know and love in humans, most of the mass of your body is assembled from proteins and encoded by 22,000 genes.
These 22,000 comprise 1.1% of the human genome, the remaining 98.9% doesn't code for proteins, and its function is not completely clear. However, most of this 98.9% is recognizable as endless repeats of retroviral genomes: Older ones almost mutated to oblivion and found in animals too, so of ancient origin, and very new ones that look almost exactly like the genomes found in actual retroviruses like HIV.
Viruses normally enter cells, make copies of their genomes, then make copies of their proteins, and assemble these into copies of themselves, then exit and the cell dies of exhaustion.
Retroviruses go ones step further: They enter cells, integrate their genomes into the genome of the cell (e.g. Your cell), make copies of the integrated genome, then copy their proteins and leave. They find it to be a good way to make more viruses at a later time. Then the cell eventually dies of exhaustion. But with HIV that can be a few years later.
Rarely, the cell was a sperm or an egg, and rarely it survived long enough for the modified sperm of egg to be passed into perpetuity - now the possession of the species.
BUT by this time, as far as anyone knew, the offspring no longer produced virus. In fact, they would not have been fitter if they could. In such new integrants, perhaps copies of the integrated virus genome are made. Perhaps even copies of the virus proteins are made, but there has never been an instance where a virus then popped out of the cell and was detected in the offspring.
Until around 2006, when XMRV virus particles were detected, by PCR, in human blood and tumor tissue. This was obviously through-the-roof exciting, because from all that "junk" in the human genome, thought to be dead virus genomes, in one case at least, could apparently still make a virus. When this kind of thing happens people jump on: What if it causes this? What if it causes that? What if it explains the other? So everyone looks for the presence of these virus particles in their "stuff".
But this leads to a second effect - all these other people first try to repeat the original experiment in their lab, as a starting point. Like, someone found that a metal detector can detect plastic coins in the ground, you have a field, and you want to try it, you first buy the same metal detector, bury the same type of plastic coins and see if you can detect them.
You can't.
OFTEN, you are a bit more careful - you try different types of metal detector, different types of coins, different depths,
You can't detect them. Shite. You talk to other people - they can't detect the virus either. The original study is not reproducible. In fact after a lot of effort by a number of different people, its not reproducible. PCR, as we know, is an extremely sensitive assay. You crank up the volume on your metal detector, and find it is detecting iron ingested by earthworms. You filter all the earthworms from the soil, bury some plastic coins, crank up the volume .. Nothing.
This happens quite a lot in science, nobody is disproving anything, nobody is calling anyone else a fraud, but a study is simply non-reproducible despite intense effort, increasing quality of effort, and multiple groups trying. Perhaps some people who invested a lot in the original finding don't want to let go though.