Can there be more than a variety of the disease alzheimer's disease?

Yes: Alzheimer's disease could be purely genetic or it can be caused by many factors. There may be also individual differences in symptom occurrence and rate of progression.

Yes: If I am understanding your question correctly, there are many different types of dementia; alzheimer's happens to be the most common.

Range mild-mod-sev: Dementia is a group of conditions marked by a decline in cognitive skills, (memory most affected ) accompanied by decreased ability to function. Different types of dementia have added symptoms based on which areas of the brain are affected. The severity of illness & its impact on the patient ( & family ) can range from mild to severe. Alzheimer's disease is the most commonly diagnosed form.

Dementias: Alzheimer's is but one dementia, and others include vascular dementias due to strokes, metabolic dementias due to hypothyroidism and vitamin deficiencies, front-temporal dementias associated with genetics, dementias secondary to multiple head injuries, multiple sclerosis, and parkinson's.

Dementias: Senile dementias, multi infarct dementias, and organic dementias such as from metabolic or endocrine diseases.