In an all-out breakdown of society 50%-70% of the US population would be dead in ~90-120 days.
1. Lack of clean water would be the number one killer
2. Everyone on daily medication would be next
Lots of people would die just because of all the dead people. Who's going to police the bodies?
After that your locale and access to resources might start to play a factor in your survival.
Clean water will be a big problem for sure, but I disagree with your time scale. Maybe in phoenix and Vegas but not in most cities. We can drink dirty water or rain water as long as Dysentery or Cholera does not take hold. It will inevitably but it will take some time.
Disease will take longer than 120 days. Civil war battle fields were highly polluted due to dead bodies, but disease was able to be managed. True smaller scale but dead bodies just by themselves are not a problem.
During the civil war, how many people had subsisted their entire lives on water from a municipal treatment facility, nearly 0%? In the USA right now, today, it's EASILY less than 5% of the population that's strictly on their own private well and has been their entire life. Our digestive and immuno-biology is fundamentally changed from 160 years ago. And again, in a total breakdown, water-treatment and sanitation, even indoor-plumbing becomes non-existent.
Think US tourists in Mexico sans bottled water, it'll be that way EVERYWHERE. You get a stomach bug, you get the runs, you get dehydrated, your kidneys shut down and you die in under 2wks. There's no anti-biotics to bring you through and your body isn't equipped to do it for itself.
