It is still flu season. Normally, the major flu season runs from October to March, but this year has been pretty nasty. There is still a significant amount of influenza being reported in all age groups and the rate of disease, hospitalization, and deaths due to flu has been higher this year than any since 2010. About 25,000 people have died from flu, so far, this year. Part of that is due to lower vaccination rates, which is just tragic, part of it is probably due to waning precautions (masking, distancing, hand washing, etc.) from the pandemic years, and part of it is probably due to just some particularly virulent stains of the flu making the rounds this year. In addition to the human flu viruses, there has been another influenza virus in the news this year, which is a strain of “bird flu”. A very widespread, very contagious strain of influenza that infects birds is the big reason that eggs have gotten so expensive. Millions of laying hens have been killed to stop the spread of bird flu, leaving fewer hens to produce fewer eggs.
We’ve all heard the phrase, “flu season”, but most of us probably never thought all that much about it. Why does the flu have a season, like baseball? Actually, I guess it should be more like basketball season, since the calendars match up better (fall/winter). How come you never hear much about “cold season” or “strep throat season” or “leprosy season” or “polio season” or “whatever-other-contagious-disease season”? The simple answer is probably that flu season is a bigger deal these days than any of those others. It is easily the most commonly-caught, potentially serious communicable disease. In the US, during a typical flu season, as many as 50 million people will catch the flu. 200,000 people or more may end up in the hospital and, on average, about 36,000 people will die from the flu and complications of the flu. Those numbers are averages and vary quite a bit from year to year, depending on how bad the flu epidemic is that year. An epidemic, by the way, is just a term for a higher-than-normal incidence of an infectious disease. It seems like “epidemic” should have a more sinister meaning, but it doesn’t. A pandemic sounds that much worse, but it is just an epidemic that has spread over very large areas, like more than one continent. Now, as I mentioned last time, our magnificent immune systems generally provide us with long-term immunity to infectious diseases that we’ve already had. We supposedly only get particular infectious diseases once, because once we’ve been exposed to a particular pathogen and recover, we will be immune to it after that. So, why do we get diseases like the flu over and over again. Every year, we have a flu epidemic. Why?
Source: US Centers for Disease Control and Prevention
Part of the answer is that “the flu” isn’t “the flu”. “Influenza” is a broad term for a disease, primarily affecting the respiratory system, that is caused by a whole Family of related viruses. There are 4 general groups (Genera) of influenza viruses; A, B, C, and D. Types B, C, and D only have one species of virus in the group, where there are many different species and stains of influenza A. The A and B varieties are the main ones that cause most of the trouble in people, with the influenza A viruses being the most problematic. Influenza C rarely causes human disease and influenza D has only been found in cattle (so far). We can get the flu year after year, but we don’t get the SAME flu year after year. The virus that causes the flu you caught this year isn’t the same as the virus that caused your flu last year, so you aren’t immune to it. Another part of the answer has to do with how the flu is spread. Like other viral diseases, the flu can’t be treated with antibiotics and, also like most other viral diseases, there are not many other really good options available for treatment. There are some antiviral drugs that, if taken early enough, can shorten the duration of the disease and ease the symptoms and, while they are not a cure-all, they may help some patients, particularly those who are particularly at-risk (the elderly, the very young, those with compromised immune systems, etc.) avoid potentially life-threatening complications, like pneumonia. The upshot here is that viral diseases, like the flu, are hard to treat effectively, so lots of people catch them and spread them.
That is one reason why we have a flu season. Another reason is the weather. If you look at the world as a whole, people get the flu all the time, year-round. “Flu season”, meaning a time of the year where there is usually a big outbreak of disease, only actually occurs in places where it is warm in summer but gets cold in the winter, as in the mid-latitudes. In places where it’s always warm (low latitudes) or always cold (upper latitudes), they usually don’t have too much of a change in the incidence of the flu from one time of year to another. In the temperate parts of the Northern Hemisphere (like where we are), the flu season generally mirrors the cold weather, running from October through May, usually peaking around February. In the temperate parts of the Southern Hemisphere (of which there isn’t a whole lot, because most of the Southern Hemisphere is ocean and most of what land there is tends to be relatively near the equator, but it includes Australia), winter runs from June to August, so their flu season is from May through October, peaking in August. There are a couple reasons why the flu (and other contagious, airborne diseases) peaks with the winter weather. One is that people spend more time cooped up inside, sneezing and breathing on each other, which gives the disease more opportunity to spread. Another reason is that the viruses that cause influenza really like the cold, dry air of winter. The viruses survive better in cooler air and they are able to float around in the air better when it’s cold and dry, giving themselves a better chance to spread.
All of that gives some idea of why lots of people tend to get the flu around January or February. But what is it about the flu that it just keeps coming back, in the first place? Why do we have really bad outbreaks of the flu from time to time which kill hundreds of thousands or millions of people? Much of that reason lies in the fact that influenza is in a fairly rare category of disease that is called a zoonotic disease. “Zoonotic” means that it is a disease that we share with other animals. Have you ever wondered why dogs get distemper but people don’t? Have you ever wondered why dogs can eat all sorts of disgusting stuff and drink out of a septic tank and not get sick? The reason is that dogs and humans don’t share many diseases. When a dog is sick with an infectious disease, it is generally other dogs that need to worry, not humans, and vice-versa. There are, however, some diseases that can directly cross between species. Rabies is one. Anthrax is one. Influenza is also one.
The same influenza A viruses that infect humans can also infect birds (chickens, ducks, some wild birds) and a variety of other mammals, particularly livestock, like pigs. In fact, the natural animal host or reservoir, of influenza A is birds. It only crosses into humans and mammals from birds. Influenza B is found almost exclusively in humans, although seals and ferrets can harbor it, too. If you see a ferret sneeze near you, you might want to keep your distance.
So, all of this should make it clear why the flu is such an issue, year after year. Any questions?
Just kidding. Let’s dig in a little more. I said before that, once you are infected by a particular pathogen, you should be immune to it, thereafter. We all get that. I also just now said that there is only one type of influenza B virus. So, why can we get the flu from the influenza B virus more than once? The answer is in the genetics of viruses. Even though viruses are not actually alive, they do have genes that code for the proteins they need, just like living organisms do. Because of the way viruses, particularly viruses like the influenza viruses, are made and how they reproduce, their genes can mutate, or change, very quickly and easily. Once the genes change, the proteins they code for will also change. The genes of the influenza B virus can mutate fast enough that, after a while, the virus itself changes enough that your immune system doesn’t recognize it anymore and you will no longer be immune to it, even if you have already been infected by it before. The situation with the influenza A virus is even more unique, because it can cross between human, bird and other mammal hosts. Here’s an example of why. A particular strain of influenza A found in birds may not be able to infect people. The “bird flu” currently found in China is an example. However, that virus is still mutating while it is in the birds, and it can, at any time, mutate enough that it will suddenly be able to pass into the human population. The same goes for “swine flu”. The influenza A virus can move into pigs, mutate and re-emerge into humans in a different form.
The H1N1 (“H” and “N” are genes for particular proteins found in all influenza A viruses. “H1” is for “hemagluttinin, type 1”. “N1” is for “Nuraminidase, type 1”) influenza A virus that caused the big flu pandemic of 2009 was a variety of “swine flu”. Thanks to a very active vaccination program, a lot of planning and activity on the part of health organizations, worldwide, and probably a lot of good luck, the 2009 flu pandemic only caused less than 500,000 deaths. I say that is lucky because the H1N1 virus is the same strain that caused the 1918 “Spanish Flu” pandemic that killed, perhaps, 100 million people over the course of about 2 years. The latest H1N1 virus was just different enough, along with the work of the health organizations, that it never got to that point. There were a LOT of epidemiologists (people who study the transmission of diseases) who were extremely worried that 2009 could be 1918 all over again. All the ads, the posters, the bottles of hand sanitizer sitting around everywhere wasn’t just hype—we could have had the most deadly pandemic in over a century on our hands.
The flu vaccine was part of what prevented a new pandemic in 2009. If you’ve ever wondered why you need a flu shot every year, that should be making more sense now. The flu viruses that are causing the flu epidemics, worldwide, this year, are not the same as the ones that will probably be causing the epidemics (or pandemic) next year. Every year, epidemiologists study which strains of influenza virus are infecting the birds and pigs (and people) of the world. They then get together and try to decide which of those strains is most likely to cross into humans and cause the flu next year. They pick the three most likely candidates, and those three will be used to make next year’s vaccine. It is called a trivalent vaccine, meaning it can protect against three different viruses. One of the choices is always the influenza B virus, since there is only the one, it mutates relatively slowly, and it is usually easy to figure out. One of the other two is an H1N1 strain, because that one is still going around in people and is, as I mentioned, particularly scary. The third one, these days, is a strain of H3N5 virus that is currently making its way through the pig population in China, and seems to be ready to break back out into the human population. Usually, the scientists get the vaccine right and it works pretty well, most years. Sometimes, however, they guess wrong on one of the viruses in the vaccine, leading to a vaccine doesn’t work as well and more people will get sick. People don’t realize how much effort and research goes into figuring out how to keep them healthy every year. This is one of the primary roles of public health organizations, like the CDC and the WHO, and they save millions of lives every year. Next time you see an epidemiologist, say “thanks”.