There was recent speculation that alzheimer's was transmissible (prion thing). The transmissibility would make sense if it were actually the fungi causing the build up of the proteins and other damage.
From what I recall, the results in a study showed that the primary care giver of an alzheimer's patient was more likely to get alzheimer's themselves. The argument against transmissibility was that primary care givers tend to be related to the patient.. therefore the more likely reason for caregiver alzheimer's is genetics.
Or as primary care giver they are exposed to the same environmental factors the sufferer is exposed to, but are generaly younger so more resistant until later life.