Dad's extremely forgetful lately. Does dementia/Alzheimer's come on that suddenly?

Any marked change in your father should be reported to his doctor. Some problems associated with dementia (of which there are many different types, anyway) can arise abruptly, but there could be any number of causes for your father's forgetfulness and it's best to get him checked out.

Yes, dementia and Alzheimer's symptoms can indeed come on suddenly; if you Google the question, you'll see the results:

What can cause rapid onset dementia?

Some possible causes include:
-Autoimmune diseases (conditions that over-activate the immune system)
-Unusual presentations of more common neurodegenerative diseases (such as Alzheimer's disease)
-Prion diseases (rare forms of neurodegenerative disease)(Prion diseases occur when normal prion protein, found on the surface of many cells, becomes abnormal and clump in the brain, causing brain damage. This abnormal accumulation of protein in the brain can cause memory impairment, personality changes, and difficulties with movement.)
-Infections.
-Impaired blood flow to or in the brain.

Vascular dementia causes problems with mental abilities and a number of other difficulties. The symptoms can come on suddenly or gradually. They tend to get worse over time, although treatment can help slow this down.

It's a good idea to get your dad to the doctor to see what's happening.
Good luck!!

You don't say if you live with your dad, or see him often? My mom and dad hid my dads ALZ for about 2 years,, he would be pretty good on the phone, and on visits they would pass things off as age related. But then things started becoming more clear. Lots of things.. so to me it seemed sudden, until it wasn't anymore, and I looked back and put things together. Same thing with my MIL, and we saw her more often.. but she had other issues. Always told she was "fine", but her hubs was not allowed to go into the Dr visits with her, even in her 80s. Got the big wake up one day when he brought me a bottle of her pills and asked what they were for... ALZ drugs my dad was on. Even then they denied it. She was paranoid to the max, forgetful. But FIL didn't want to know the truth because she was "good company" So I agree with checking with his Dr, if he will let you.

My grandmother had a series of strokes that descended her into full blown dementia in 2 days.

Get your dad checked immediately for any possible medical conditions that could be causing this, early detection can get better treatment and better results.

I agree with Pamzimmrrt... couples who have been together for a long time can cover for the other one for an incredibly long time... until they can't any longer. That's likely the case unless there's another etiology for it. Get your dad evaluated asap so they can diagnose and treat whatever is going on with him. If it is a stroke, Mini strokes are warning signs of the big one to come. Better safe than sorry. Best wishes to you and your dad!

It may seem sudden but I have read where Alzheimer victims have hidden signs and symptoms for as long as 10 years.
I know with my Husband when I noticed something a miss I thought back to other incidents and gradually saw a picture unfold. ( often said Alzheimer's should change symbol from a purple ribbon to a purple puzzle piece much like the Autism symbol..it is not until you put all the pieces together that you get a full picture of what is going on)
All you can do once you have the diagnosis is accept that this is how it is going to be.
Prepare yourself
When things get worse..anticipate the worse and rejoice if it is not as bad as you imagined it could be.
Remember that a "bad day" this week will become a "good day" in a few months..and that will continue to be the pattern so learn from your bad days.
Decide early if and when you will have to place your Loved One. My only criteria was safety. If it became unsafe for me to care for him I would have to place him, if it became unsafe for him I would have to place him. Luckily I was able to keep my Husband at home.
Research all the help you can get. Is he a Veteran? if so there might be a little or a lot of help from the VA.
Talk to an Elder Care Attorney as soon as you can, make sure all legal and financial aspects are covered.
Hang in there, you will learn a lot and pass on what you learn because every Alzheimer's victim is different.
Find a good support group.

There are different types of dementia. Alzheimer's dementia typically is a slow steady decline while vascular dementia is characterized by a sudden decline followed by stability then another decline in memory. This is usually a result of a T.I.A or strokes over time. However, for any sudden mental or physical decline, I would recommend an evaluation by the primary care physician. He or she can the refer your Dad to a neurologist, psychiatrist, or geriatric psychiatrist for a full cognitive evaluation. The physician may also wish to order laboratory and/or imaging studies to investigate cause. Good luck.

In the case of my father (age 85 at the time) he seemed to be getting forgetful at times. Two months later my mother called and said, “I don’t know what’s wrong with your father but he just walked out into the garage and is just standing there”. I got there and he seemed to have difficulty walking and seemed “spaced out”. We took him to the him to the ER (on Memorial Day weekend) and were there 6 hours. They took and x-ray and discovered he had what is called an acoustic neuroma (or non cancerous tumor) in his brain next to his ear. To make a long story short, this tumor was apparently causing what is called “normal pressure hydrocephalus” or fluid on the brain. He ended up getting a shunt which alleviated the pressure and in about a month after the operation seemed just about normal again. However, a couple of years he started getting dementia again following a pacemaker replacement operation. He eventually passed away at 89. Apparently other things can cause sudden dementia like symptoms also.

You should have him checked first for infection especially urinal infection

You aren't giving much information. Stress can also affect memory without having any dementia.