Infectious diseases have the capacity not only to influence the host population but also to interacting species like predators. In particular, they can reduce host densities, which can have knock-on effects on predators. Here, we consider how an infectious disease in the prey affects the predator-prey relationship where the prey exhibit some kind of group defence against the predator (using a Holling type IV functional response). We find that the disease can reduce prey densities to levels where the group defence is weaker. This weakened group defence allows predators to survive in many situations where they could not without the disease.