Life expectancy is an estimate of how long a person is likely to live. It is difficult to give a single number as the answer, as it depends what country we’re talking about, how old the person is, whether the person is male or female, and other parameters.
But, let’s throw out some numbers anyway.
How long do people live?
The average life expectancy of a newborn baby is currently about 67 years. This means that each baby born today will on average live to about 67 years.
That number might not sound very large, but it’s more than twice as long as in any other era of humanity. Even just one hundred years ago, life expectancy was only 31 years! 
This number does not mean that a person that is born today is constrained by the number 67 and will definitely die at that age. Life expectancy is what we currently expect a person to die at. This changes over time as we develop new medicines, better understanding of how the human body works, and better surgeries to replacing aging or faulty organs. We are not constrained by the limits of medicine at the moment of our birth. We can expect to take advantage of new medicines as they develop through our lifetimes. 
There is a very important distinction to be made here.
The above numbers are average numbers, in that they take into account all people of that time, including those that take care of themselves, those that don’t take care of themselves, those that live in wealthy countries, those that live in poor countries.
It makes sense to say that a person who lives in a more wealthy country and takes care of themselves, will live longer than a person who lives in a poor country and doesn’t take care of themselves.
Because life expectancy increases over time as our researches figure out more and more, the longer we live, the more likely we are to live even longer.
For example, let’s consider diseases such as HIV/AIDS and cancer. in the 20th century, being diagnosed with either of those was virtually a death sentence. But today, there is a “functional cure” for AIDS, and there are a number of potential cures for cancer also being worked on. All you need to do is survive long enough for the research to find better solutions, and those diseases will no longer be a concern for you.
Preventing the common causes of death
Currently, the top five killers of humans are: 
- heart diseases (cardiovascular)
- infectious/parasitic diseases
- heart diseases (ischemic)
So, logically, if you try to avoid those problems, then you should be much less likely to die in any particular year.
This means exercise, avoiding or cutting down on toxic habits such as alcohol and smoking, and avoiding stress. 
If you are serious about living forever, then you should read about each of the above causes of death, and try to change your habits so you are less likely to die of them.
If we add Quantum Immortality into the conversation here, then you could say that you don’t need to make any change at all to your habits, as you will survive everything that is thrown at you, through sheer statistics.
But, there is a very big difference between “surviving” and “living”.
Heart attacks hurt. A friend of mine, who had his first heart attack at age 32, described the pain as “like a truck sitting on my chest”. He survived that heart attack and at least one more, and is currently 52 years old and very healthy, but I’m certain he would have preferred not to have had the heart attack in the first place!
I have lost a few friends to cancer. In some cases, the cancer came and took the person within a year or two of diagnosis. In one very lucky case, the cancer (leukemia) took nearly twenty years to take the person even though the doctors said she had only a few months to live.
Cancer is not currently curable, but it is certainly preventable.  There are a number of potential cures being worked on at the moment, so Quantum Immortality says that you will survive cancer long enough to get cured. Or at least long enough to go onto a course that maintains your current state and then eventually you can be cured. But, considering that cancer is preventable in most cases, it is better that you don’t get it in the first place.
A look at Influenza
It’s worth looking back in time at how the list I wrote above has changed.
In the early 20th century, the number one killer was infectious diseases such as influenza. Infectious diseases is currently the number two killer, but Influenza is much less fatal now than it was then.
Influenza still kills tens of thousands of people every year, but as we learn more about it, it becomes less and less dangerous.
The worst deaths occur during “pandemics”, when fatal forms of influenza evolve and kill a huge number of people before we figure out how to solve the problem.
There were three quantified pandemics during the 20th century. Each of them killed less people than the pandemic before it on the list:
|1,000,000||1968-1969||Hong Kong Flu|
One of the major reasons for the decrease in numbers is vaccination.
Vaccination involves injecting a dead version of the virus into your body so your immune system can figure out how to defend itself against it. Then when the real thing comes along, you might get sick for a while, but you are much less likely to die.
There has so far been one pandemic in the 21st century, called the Swine Flu pandemic.
Only about 16,000 people died this time. 
I think the most important thing to take from this is to realise that the effort that we (as a race) put into solving the most common causes of death, is paid off with an increase in longevity in the general human population.
We live much healthier lives these days than people in the early 20th century, and it’s not like we actively put any effort into it – it’s almost like we “absorb” healthy lifestyles by osmosis. Our friends exercise so we exercise as well. Our friends drink less or smoke less so we do the same.
Our environment is improving, and we are improving along with it.
In this chapter, we learned that humans are on average living twice as long now as they have ever lived before.
We also looked at the top five causes of death, and how prevention of these causes is actually not all that difficult.
We also looked back in time at how one of the historical number one causes of death is not such a huge deal these days because of medical advances.