why do flu epidemics, like 1918 one, end?

ok, so here is a quote from Wikipedia article on 1918 pandemic:

So could somebody please clarify how this makes sense? E.g. if the new quarantine policies helped, why didn’t the epidemic continue in places without effective medical system and government, like maybe in Africa, Middle East and the still pretty disorganized China?

In terms of the strain mutating to become less lethal, suppose we have a million people infected. Does the strain mutate in all of them simultaneously so that further infections from them are not lethal? Shouldn’t the mutation sort of split the strain into several, with some of them unchanged and as lethal as the original and others changed? How does this work?

I presume that once a virus is lethal, word gets around and those who have it are quick to self-quarantine, while those who don’t take additional precautions. Those who survive it and are on the mend may pass it, but presumably they have a less virulent strain. Wash, rinse, repeat, and it eventually gets even less virulent.

It doesn’t do the virus much good in the long run to kill the host unless it has a long incubation time. The viruses that tend to kill will make you feel like crap pretty quickly such that you would know you were infected and thus take yourself out of the pool of potential contaminators pretty quickly.

I’m not seeing any mention of quarantine in your quote.

A single virion (ie one individual virus particle) mutates. That virus then goes off and infects someone who, because they don’t die, then spreads it to dozens other people. Just because the person feel OK to go shopping, for example, doesn’t mean that they aren’t shedding the virus. However if a person feels dead it’s a safe bet that they really have stopped shedding the virus.

So while the original strain virus remains infective for a few weeks, and can only readily infect those nursing the sick for most f that time, the less lethal form can potentially remain infective for months, and during that time can be passed to multiple strangers. As a result the mutant strain infects orders of magnitude more people.

However the two strains are still very closely related, and once a person has fought of one strain they are immune to the other. Because far more people are infected by the mutant strain, the original lethal strain is rapidly outcompeted, and eventually becomes extinct.

So yeah, the virus does split into lethal and non-letahl strains. But any individual host can only ever be infected with one or the other, never both. Since the lethal strain will infect far fewer hosts it very rapidly becomes extinct.

Several things occur to end epidemics including enough immunity to the original bug to slow down spread but the part that is being asked about indeed is as Blake explains and refers to the fact that a less lethal version will outcompete a more lethal one.

Circumstance A: Highly and rapidly lethal bug. Subject A gets it and quickly gets so ill that he is bedridden and then fairly quickly dies. Even if the bug has a high degree of transmissibilty for a given level of exposure fairly few have had a chance to get that level of exposure.

Circumstance B: Newly mutated and/or reassorted version of the same bug just as catchy but less severe. People stay alive and even out in the world functioning for prolonged periods of time coming into close contact with a wide variety of other people, coughing in their faces, etc.

Which one will spread more and faster, and by so spreading help decrease the number of those susceptible to the original version?

The competition may even occur within individuals. Turns out that we are not generally infected with a strain, but a whole little group of strains that are traveling together reassorting along the way. The deadlier strains attach better to the lower respiratory tract and cause more serious problems, but they do not spread to others from there as well as the ones that attach preferentially to upper respiratory tract cells.

Being less than quickly fatal is an evolutionary fitness advantage for a pathogen.

Just a note of thanks to expert posters such as Blake and DSeid for making it easy for a layman to understand. You guys make it clear why belonging to and using this board for information is my first choice.


My wife read (and I think this was speculation, but it makes some sense) that flu tends to become less lethal for basically the reason Blake gave. People who feel worse stay in bed, and people who are less sick are more likely to be out. For the 1918 flu, though, WW1 was going on, and many of those infected were solders. The people less sick stayed at the front, but the sickest were sent home, via train, where they could infect more people. I don’t have a cite, or know if there’s any actual evidence for this idea.

One theory was that the main reason the flu was so virulent had little to do with the virus and more to do with nutrition and general health. The flu hit army camps, and army food is not the best; in addition, all the training and hard work weakened you quicker if you weren’t on a good diet. In Europe especially, there were many food shortages, not to mention people wounded in the fighting.

If this is a factor, it would also explain the end of the epidemic: the weak were weeded out. Actually, that explanation is also a good one: those who were at risk of dying from the virus did so; those who had immunity did not.

If you have millions of people infected, won’t there eventually come a time when nearly everyone has been exposed to the virus?

Sure. But consider the facts: 1918 flu had an initial mortality rate of >10%. The population structure at the time was heavily skewed towards the young. The disease mostly killed young people. That would mean that, for the epidemic to end in that manner, we would have to see at least 300 million people dead. The death toll during the first few months was on track to exceed even that target.
But the highest estimates for the final death toll are in the range of 50 million, or about 3%. Most estimates put it much lower, at around 1%.
So something else caused the epidemic to end long before all potential hosts were infected.

I read a book called “The Great Influenza” that made the same claim about army life, except that the theory was that it was an exceptionally cold winter, and guys were huddled together in small tents with inadequate heat. Actually, the book makes a convincing case that the first cases were actually in an army camp in, if memory serves, Kansas.

More recent thinking (following SARS, H5N1 and recovery of well preserved 1918 pandemic victim tissue) places the blame for young adult deaths in 1918 on Cytokine Storms - a positive feedback loop in the immune system that causes the lung inflammation that eventually kills the victim. A healthier young adult victim has an immune system strong enough to kill them.


Good explanations; plus, once enough people have acquired immunity, the number of people getting sick and still spreading the virus goes down to the point where the odds of an infectious person encountering a non-immune person is rare enough that the disease stops spreading… until a new generation of non-immunes grows up.

We saw the same cycle with the Bubonic plague or Black Death. The epidemic from Asia killed somewhere around one third of Europe around 1350. It came back every few decades in succesively less virulent forms.

We probably are seeing the same thing today with AIDS. Years ago, people were dropping like flies as the disease swept around the world. Sickness and death followed infection within a year or 3. Today, the norm is to live with the disease without an outbreak for decades. Partly this might be attributable to the number of people on the drug regimen, but the mutation issue probably also comes into play; the ones who lived long enough to infect many more are probably those with the successively less active forms of the disease.