There is "compelling evidence" that people with Alzheimer's have a variety of fungal species in their blood, according to the study published recently in the Journal of Alzheimer's Disease.
Brain samples taken from dead Alzheimer's patients also "clearly indicate" the existence of fugal proteins.
This means those fungal infections had invaded the central nervous system.
The Spanish team behind the study also say they found various fungal species in single patients depending on the severity and evolution of the disease.
All this paves the way for the possibility that fungal infections could help cause the disease or be a risk factor, said the scientists from Barcelona's Severo Ochoa Centre for Molecular Biology and the Carlos III University of Madrid.
"Future studies could help determine the role of fungal infections in the appearance of development of Alzheimer's," they added.
Alzheimer's disease is one of the most common form of dementia. Affecting mainly the elderly, it has no cure. Symptoms include short-term memory loss, confusion, and mood swings.
Some 35.6 million people were estimated to be living with the disease in 2010, according to the World Health Organization.