Why are so many people are getting dementia & alzheimer's now?

People live longer. By far, the largest risk factor for alzheimers is age. Roughly 1 out of 100 of those aged 65 have it, 1 out of 20 of those aged 70 have it, 1 out of 10 aged 75 have it, 1 out 5 aged 80 have it and nearly half of those older than 85 have it. This is exponential growth by age, so as life expectancy has lengthened, there has been a rapid rise in those with the illness.
Awareness. It is not fully understood why there is so much now. In part is because awareness of diagnosis or recognition of the disease is earlier then before. The real cause of alzheimer's is not known but there are entities called prions that cause mad cow or jacobs dementia. Alzheimer's has clear cut changes in the brain (neurofibrillary tangles, senile plaques, neuronal loss, deficit in acetylcholine.
No. More aware & index of suspicion higher & more frequently diagnosed.