Possible timeline of death in a world-wide post-collapse scenario
Three days to one month
People need water in three days. As their thirst increases they'll use supplies that are pure, but then of lower and lower purity based upon the thirst mechanism. Remember this, people don't drink water very well based upon the thirst mechanism. Many are disconnected from understanding when they are dehydrated today. It's only because the live in a post-modern society with running water and supplies and don't have to collect water so that they survive.
As one dehydrates, one begins to have lower cognition. Critical thinking declines and any water source will be deemed as “tolerable”. As this happens, impure water will be consumed. Pollutants from contaminated groundwater will kill some, but many will continue to survive for a time. Amebic dysentery will affect many from coliform bacteria from animal or human fecal matter. Many deaths occur from this from cross-contamination, followed by cholera and a host of immune system insults. Generalized weakness accelerates in the host, and those people lose the ability to care for themselves and for their family. The weaker members relying upon stronger ones to gather water will die: elderly, infants, toddlers, older children, etc.
People need shelter to protect themselves from exposure especially in Winter. If it happened today November 14, 2012 in temperate climates in North America, and if people bugged out from urban areas in a mass exodus, we could expect to see many people to fall prey to violence, dehydration, theft of supplies leading to malnutrition, depressed immune system responses from the cold, outright exposure from lowered temperatures without adequate clothing or shelter or firemaking ability, or a combination of all of the above.
Many people have prepared 72 hour kits to help them transition to safer areas, but don't have hunting, trapping, or fishing ability. Even if there were lots of wildlife to gather, so many new hunters will cause animals to evade them or the successful ones will overhunt the wildlife areas and kill them off.
Unless they have the experience of living in the wild and avoiding exposure then they will probably die within a week. They will get weaker and weaker as they travel without a way to supplement their blood glucose to maintain consciousness. They'll get sleepy and sloppy. Mistakes will lead to getting lost, falling into a ravine, get injured and can't walk or break a bone, fall to predation from other bugging out people.
People can survive up to three months without food as long as they are sedentary. If not, and looking for firewood, water, and other activities, then they'll die sooner.
Bugging out folks looking for shelter in less urban areas will be turned away from the towns. They won't be needing people unless they have very high skill levels of rare abilities. Some will be violently repelled.
People on medications (who haven't filled their prescriptions for three months) will begin to have less and less medication in their blood. The decline of the medication will begin to show effects in as little as a week. Renewed blood pressure issues will suddenly increase especially under increased work load. Many heart attacks occur. Many who don't have blood thinners will begin to form micro-clots and many of these thrombo-embolisms create strokes. They become paralyzed on one side and therefore unable to care for themselves or communicate. They throw a clot, like a saddle embolism, and can't breathe.
Others who can't gather herbs to control blood sugar begin to see higher and higher blood sugar when they can test it. Eventually they won't be able to test it and must guess. The ones with Type I diabetes will die based upon the natural progression of their illness if they already have circulation issues or eyesight loss. Both of those pre-existing conditions will lead to less foraging capability. The less healthy will die, then the older diabetics.
Mass die offs in nursing homes. Many elderly die in their homes.
Many people are barely surviving based upon lack of food. A great extinction has occurred from all of the above. Some who have survived have looted from stores to supplement their food supply, or found materials to help them acquire a food supply. Some will have died in the process as it become similar to Nature and the survival of the fittest.
Gangs will probably die unless they adapt are lead by intelligent leadership. New gangs will form based upon a desire of using numbers to overwhelm prepared people with supplies.
In order to attack prepared people or liberate supplies, gang members will need a lot of calories far exceeding their normal diet. It's very heavy activity to move around that much searching, standing watch, scouting, fighting, etc. Many will be unable to acquire enough calories by this method and will die. Cannibalism is to be expected.
Many people have died from lack of medications. The ones who made it three months were in a second tier of younger more healthy people who were able to adapt but had supplies and clean water. Eventually though they ran low on supplies, got dehydrated, illness set in, and were too weak to gather enough firewood to cope.
Many died from ingesting what little food they could find that was of very low quality. The food was either spoiled, ill-prepared and caused bacterial infections, was low in calories, wasn't really edible (some will try eating what grass they can find...it happened during the Ethiopia famine). Some will have extended flour with things as fillers that really isn't nutritious. In history this often is sawdust. Many will have eaten rats, gopher, or other creatures that had disease of one kind or another, and so they became weakened and sick from ingesting it.
As food becomes unavailable in urban areas, many will leave on foot. Gasoline will have been used either to heat or to attempt transportation away from the areas or for power generation. Many of the weakened people attempting to leave will die having no survival skills in the wild and fall prey to the same events as the first tier.
Up to 90% of the populace have died. Those with supplies ran out of food slowly, but had no idea how to garden or hunt or trap or fish. They attempt it, but are weakened. There's an increase in animal activity and the ones who are more successful acquiring animals will make it.
There are no obese people anymore. People's faces are very gaunt and drawn. There's a lot of bitterness. Most of the depressed people, schizophrenics, drug abusers, etc have died. Many became suicidal. Many in effect became suicidal since they stopped self-care and slowly declined and then gave up.
Many children have died. A reduced ability to find food, accidents, illness, dehydration, etc have killed them. A lack of parents or guardians means that few coped. Some found help, but as food stores ran down, they were turned away. It will follow historical patterns.
The die-off of so many people without burials will lead to eventual groundwater contamination. Waves and waves of epidemics kill survivors. Very few people who bugged out have survived unless they went to safe places with long term food supplies and are planting gardens.
Many new gardeners cannot produce enough food to save themselves. There's nothing left to hunt, fish, or trap anymore. This causes changes in the forest and meadows. Animals retreat into the wild areas. Eventually their numbers will increase based upon the lack of predation there. Only people who take long hunting trips will be able to acquire them, but at great risk of returning to their tribes.
A host of nutritional deficiencies leads to blindness, weakness, secondary bacterial infections, parasites, etc. These kill many.
Most of the gangs have died. They only had power as long as they had weapons and strength. Unless they had long-term skills to create food, then they can't make it from foraging upon survivors. There are less and less survivors on which to prey upon, and the lower success rate means that many gang members turn upon themselves.
Few people can preserve food adequately. Much of the food is eaten as it is harvested. Starvation occurs in a strong way among survivors without supplies to pull from. Many of the strong ones die.
95% of the population has died. What few people remain have very high skill levels, have learned to preserve food by extraordinary means, have raised livestock to provide routine protein. They have clean water. Their homes are warm and safe. They have weapons other than firearms to protect themselves as most ammunition is gone.
Much of the remaining animals have departed from any areas formerly occupied by humans. Those that do remain if they survive the winter may repopulate the areas slowly since there's no longer that many humans there. Many of these areas have high bacterial counts in the water, but those animals who are strong and have adapted will survive it.
Fewer humans means less possibility of finding marriageable partners based upon finding love. People will become less and less picky based upon finding providers (bunkered people with tons of supplies) and those with very high skill levels.
Some communities have pulled together. These may be very few in number, merely surviving families of preppers. Some intermarriage begins as not only natural attraction, but also alliances strengthen the community.
Smart communities recruit from nearby survivor communities. They look for those with technical ability outside their own.
The ones who make it have preserved old knowledge and use it daily in creative way to increase security. This means some replanting of forests is occurring, wells have been dug and most likely cisterns. All have rabbits and goats and probably some with bee operations. Some milling of flour is happening. Some ethanol production is in operation. Some generate power from hydroelectricity and solar.
Most heat using rocket mass stoves unless they have lots of firewood available. Still it's the most likely scenario based upon protecting wood for tools and lumber. Many use solar ovens in the summertime. Some biogas operations in warm climates that create enough average temperature to produce methane efficiently or at least seasonally. Most tribes have wood-gas generators.
Many will be harvesting materials from their environments in order to repair and maintain equipment.
Some peddling will occur. These people move from area to area creating trade networks. The collect things like salt from the coast and move inland to trade for locally produced items or found items like tools, sewing needles, textiles, shoes, etc. Fuel will be unavailable so they will operate by having horses to transport themselves or they will more likely travel on rivers.
This means that those places around rivers will act as hubs. Some trade will occur based upon proximity, and then smaller trades based upon the movement of the peddlers. Some go-between people will profit by trading.