1918 Spanish flu and pre-1889 flu
Let’s start with numbers. Spanish flu plagued our Earth back in 1918, infected 500 million people and claimed an estimated 50 million people. Of those who lost their life, a significant percentage of them were healthy young adults. It is an odd flu. Most influenza outbreaks would cause mortality in juvenile, elderly, and immunocompromised patients. Spanish flu did the opposite where 50% of the lives claimed were the healthy ones.
One important question needs to be addressed here. Why this flu killed predominantly healthy people?
If virologists (scientists that study viruses, especially the ones that make you sick) want a Christmas present, it would be the answer(s) to the question. But scientists know that Santa does not exist, so they have to fork their own effort and money to find the answers.
The first theory we arrived at is the activity of the immune system.
The immune system is like an army: very strong and composed of many parts. Here we have combatants and weapons. The combatants are immune cells (we call them lymphocytes) and the weapons are cytokines.
In healthy adults, the cytokine reaction is matured and strong. It means when the army of immune cells bumped into a potentially invasive pathogen, the weapons they wield are powerful to knock down the infection. Babies and elderly have the same weapons, but their potential is not maxed out or reduced.
For this odd flu, somehow the weapons (cytokines) were out of control. Under normal circumstances, weapons should be stowed away once the pathogen is no longer becoming an imminent threat, but in this specific case, the army of immune cells overproduced cytokines to a point the cytokines started to damage the host. We call this phenomenon as the cytokine storm.
It kind of makes sense now because healthy young adults have powerful weapons, and when powerful weapons fall into wrong hands, bad things can happen.
Well, that explains the pathological side of the infection. Pathology here means how a disease progresses in the infected individuals. Since back then the field of biology was really its infancy, we could not perform a real-time investigation of the disease.
And here we are stuck with one more question. Could it be something else that contributed to the pathogenicity of this Spanish flu in healthy people aged 20–40? Looking at the graph below (courtesy of Jeffrey K. Taubenberger, 2006), patients aged 45–64 were less susceptible.
According to an article on The Atlantic published on November 2016, it appears that we might have solved another factor that potentially contributed to the disease pathology. Let’s go back to 1889 and what happened before that.
It was reported that the Spanish flu which occurred in 1918 had a connection with a certain kind of flu virus that hit before 1889. Let’s call it the Spanish-ish flu. Okay, that sounds a little bit awkward. Let’s call it the pre-1889 flu. What makes this pre-1889 flu interesting is that this flu appears to have similar structures like the 1918 Spanish flu, but not as deadly. For most people, it was a regular flu. Those who survived had antibody against pre-19889 flu virus in their blood, and they lived couples more years.
When the Spanish flu hit the world, apparently the antibody against pre-1889 happened to confer some degree of protection against the Spanish flu virus. It is quite unfortunate to those who never encountered the pre-1889 virus, otherwise, they might fare a better chance to live.
These theories here sort of explain the question stated above but we do not really know for sure. Studies may suggest, but sometimes the reality could offer answer one would not expect.
