http://www.dailymail.co.uk/news/article-3413873/Alzheimer-s-cause-discovered-Poisonous-algae-freshwater-lakes-reservoirs-UK-fuelling-dementia-epidemic-afflicting-1m-people.html?ITO=applenews

Scientists have discovered a type of algae that produce toxins thought to cause dementias. Up til now, old age and genes are thought to be the most determining factors involving onsets of Alzheimer's, but now scientists believe this toxin plays a big role in it.

The algae producing the toxins are found in feshwater lakes and reservoirs across Britain, but the toxin travels up the food chain. Oysters and mussels, for example, contain this particular toxin.