Don't get me wrong, it's a terrible disease. However, "deadly" typically denotes something that will kill you. There are many other terminal illnesses that claim more lives than Alzheimer's. There isn't much of an argument to be made putting Alzheimer's at the top of the list in terms of deadly diseases,
Alzheimer's disease is an insidious blight that destroys brain cells and causes erratic behavior and memory loss. It "kills" the person we know long before their body functions cease, due to the erasure of their personality. But Alzheimer's is far outpaced by more commonplace causes of death like heart disease or respiratory infections. Those types of diseases may not hurt the family of sufferers as much as Alzheimer's, but they do cause more death.
There are diseases that are more deadly than Alzheimer's, but few that are as heart breaking for family and loved ones. Every person that gets Alzheimer's will become stricken with memory loss, and eventually a shutdown of the body's organs. Yes, there are still many more deadly diseases when it comes to heart disease and cancer; but, Alzheimer's is especially cruel because it robs one of his mind; stealing his personality and life.
Alzheimer's is a heart wrenching, debilitating disease but there are far more deadly ones. Lou Gehrig disease, muscular dystrophy, cancer etc are arguably more deadly. You have all of your mental faculties while losing all of your physical abilities and watching yourself die. At least with Alzheimer's there are moments where the mind is released from this agony.
Alzheimer's definitely seems to be unpleasant, but I do not think it's the most deadly disease. The various medical articles I have read have indicated that Cancer is the most deadly disease. And while new discoveries are being made to get sufferers of Cancer into remission, research also seems to discover new triggers of getting cancer. Exposure to various waves, things we eat (whether we know about it or not), or plain old genetics all seem to possible ways to get cancer later on. And then there are the triggers that we don't even know about. Hopefully sooner than later scientists will be able to learn more about both of these horrendous diseases and offer solutions that will prolong life.