Your above statement isn't true! Those diseases are not modern.
Alzheimer's (Senility/Dementia) - The term Alzheimer's was coined in 1906 and senility/dementia had been studied for some time prior.
Diabetes - "The term diabetes was probably coined by Apollonius of Memphis around 250 BC. Diabetes is first recorded in English, in the form diabete, in a medical text written around 1425."
History of Diabetes
Atherosclerosis - "Considering the morphological findings in egyptian mummies at the beginning of the 20th century, atherosclerotic lesions were also apparent in pharaoh mummies more than 3500 years ago. Hippokrates (469-377 b.c.) described the sudden (cardiac) death, whereas Erasistratos had documented the typical claudication intermittens symptoms of peripheral arterial disease approximately 300 b.c."
https://www.ncbi.nlm.nih.gov/pubmed/11680058