That's why that 49% figure is so exciting. Because it's so high it could mean that, say, this retro-virus truly is involved in 100% of cases of true schizophren

That's why that 49% figure is so exciting. Because it's so high it could mean that, say, this retro-virus truly is involved in 100% of cases of true schizophrenia, but that those other 51% of the people either (a) don't really have precisely "schizophrenia" but a slightly different disease that just has very similar symptoms, and/or (b) experimental error of some kind. And not only could there be different diseases that have very similar symptoms but it's possible that a single disease could be reached via a different set of initial/contributing causes, or at least a slightly different causal pathway. For example, take some hypothetical disease that is found to be "caused by" a lack of vitamin D. Well, it may be possible that a particular person gets plenty of vitamin D intake, but that they have a gene variant that makes their body not utilize vitamin D normally. Or perhaps their genes are fine, but they have some other gross bodily malfunction (organ failure?) that is causing the vitamin D utilization process to be disrupted or minimized. Regardless of the specific chain of causation, they could all result in the same unhealthy symptoms expressed.