Honestly? Nuclear power plants left to melt down.
Think about it. Even with redundancy after redundancy built into those things, safe guard after safe guard.. ((Not nearly as many as we'd think, nor nearly as fool proof)) After a few weeks to a few months, those things are going to start melting down. Look at where they're situated. Now remember how the jet stream curves down and bends up.
If you're out side of Montana or North Dakota, maybe New Mexico.... you're pretty much fucked.
Past that, people don't understand gasoline. They watch post apoc movies and see people scrounging. Driving around. Finding gas. Filling up their cars. hell even Mad Max and other post apoc movies and stuff show people driving.
In reality. That's not going to happen. Gasoline is an incredibly refined substance. It doesn't come out of the ground that way. It takes alot to get from what comes out of the ground, to what goes into your car. More over it doesn't 'sit' well. WIth out special chemicals in it, gas goes 'bad' pretty quick. Parts of the chemicals in it evaporate bla bla bla lots of science, but yeah, after 3 months of sitting, the gas is going to start to sour and go bad. 6 to 9 months and it's going to be problematic in some engines. 12 to 18 months and it's going to be pretty funky and won't work in all engines. 2 years+? I hope you got a bicycle. Now that's not a 'danger' as much as it's "Something people don't think about and will count on, when they shouldn't/can't" Gas moves our cars. Runs generators, etc. Getting away from zombies sucks if all you have is feet, or at the best, a bicycle. Now. I'd use a bike, but you can't exactly plow through a herd.
Disease is going to be a big one. Hundreds of thousands to millions of dead people will bring disease back in a huge way. Especially when millions of them walk around to spread it. Doctors are few and far between THESE Days. Now imagine if 1 in 1000 people are alive, what are the statistical chances of them being a doctor? Low. Made more so, because the hospitals and the cops die first in a zombie rise, as people injured are taken there, then zombies pop up and bite them, or they're swamped by the injure and dieing who.... become Zombies and yeah. Doctors are going to be some of the first to go. So any sickness is much much worse. People used to die from the flu. People can die from stepping on a nail. An infected cut can kill you. Giving birth is STILL one of the higher percentages of things that kill women. Try doing it in a cave like we used to. A broken arm or leg could easily kill you. The cities are going to be crawling cesspools of sickness. ANd they have most of the supplies.
"Other People" Will be the most 'immediate' threat. "Fight the dead, Fear the Living" is an apt thing. Even if you don't watch "The Walking Dead" you gotta know that the living are going to be much more dangerous on a one on one number than zombies. People may want what you have. No law and order, they might kill you to take it. Be it your gun, your food, your woman, your car. Even people you wouldn't expect it are going to be functioning on animalistic type mentality. That little girl you find hiding in some building might seem harmless until she cuts your throat as you sleep because the LAST people she ran into raped her for a week straight. The little geeky guy you take into your group, might want your woman, or just your gun. And... picks up a rock while you're sleeping and brains you. That's before you even encounter those people that go "Chasing down a deer is HARD..... Walking up to another survivor and going "Oh god I'm so glad you're here! We thought we were the only ones!! STAB STAB STAB STAB...... hehehehhee.... Make a fire... we're having a BOB-b-que!" is easy and EAT you.
In a Zombie rise at the start, taking in people would be pretty instinctive. Humans are pack animals... these days we're more herd animals but.. still communal. After a very very short time, it's going to become readily apperant, one of the most dangerous things you can do, is trust someone you don't know.
Weather would be a danger, but usually in conjunction with the previous things listed. We take power for granted. Sure people survived before electricity, but they grew up knowing how. Anyone above... DC or so and north... they're in trouble if they can't get south, QUICK. Even going south you have to put up with stuff. Look how bad the country freaks out over realitivly mild weather. Atlanta last year, LOST THEIR FUCKING SHIT, when they got like 2 or 3 inches of snow. People stoping cars on the interstate. Sleeping in schools, sleeping in stores. HERMERGURD!!
It was insane. Made the news for weeks.
Where I live, we just laughed and laughed and laughed. 3 inches of school? Oh must be Tuesday. OUr school buses don't slow down for less than 9 inches or a foot. Much less everything shutting down. 3 inches doesn't stop a car, unless the driver stops it. lol. Cars weigh 1000s of pounds. But if you watched the news, the entire state shut down. State of emergency declared. Kids left in schools for 2 or 3 days, for snow, less than a finger deep.
Imagine that sort of thing, but with no power and no heat. "Oh we can burn wood" Yeah? How many people actually have the means to harvest enough wood to survive a winter?
How many people have enough knowledge to clean water?
Hunt for food? REALLY? How many people can actually hunt enough to survive NOW, with cars and ORV's and high powered rifles, and stuff. With out having to dodge 100s or 1000s of undead? Very very very few. "Oh I take a deer every year!" Thatr's cute... can you take one a week? year round? Cuz that's about what it's going to take to keep a family of 4 alive.
There's alot that people don't really think about in a Zombie rise.
Heck reproducing is astoundingly dangerous. Just being pregnant can kill a woman in 100s of ways. Giving birth is very dangerous at the end. If she survives that.... you have a baby. "Oh I'll breast feed!" Great. If the baby takes the boob, and your milk isn't messed up by constant stress. How well fed is the mother? Breast feeding takes ito ut of you. You could end up getting sick and both dieing. More over.. .baby's cry. Sometimes they just cry. For no reason in warm homes with air conditioning or heat they cry. And it's loud. and it carries. ANd you can't stop them till they want to stop. They'll cry around binkies and scream around your hand. What happens then? Every zombie with in a mile zeroes in on you. With out power and all the sounds of the city/mankind to cover it, such a noise is goign to carry a long long way. It's been biologically designed to from millions of years of evolution. Can you imagine being pregnant during a rise? Or having a screaming child with you? Yikes.
I could go on and on and on.