What would a multi variable model look like where, say, both herpes and plaques contributed to Alzheimer, but you couldn't find proper correlation with either t

What would a multi variable model look like where, say, both herpes and plaques contributed to Alzheimer, but you couldn't find proper correlation with either taken individually? There are plenty of diseases where we have identified multiple risk factors successfully. We have examples of diseases where either risk factor alone is often causative (e.g. pulmonary cancer can be caused by smoking, or by long term asbestos exposure, and if exposed to both, fixing just one won't make the risk very low; and there are of course many other substances that can cause this). And we have also identified diseases where you need two (or sometimes more) separate factors to trigger it (e.g. shingles is a diseases that is triggered in individuals that previously had chickenpox, but generally only if their immune system is weakened by something else, typically old age).