I think it’s just that numerous viruses are in our bodies, but are held “dormant” by our immune system. But in weightless space their immune system becomes weakened and those latent viruses jump up. Herpes is a virus . Every one has cancer in their bodies occasionally and it is killed off by the immune system. But sometimes the immune system is weakened or just gets overpowered.
Well they also have higher rates of cancer too, partly because of higher radiation and also the immune system is weaker and may not take out the cancer cells as they could in normal earth gravity and more radiation protection on earth.