This is what immunity is not: You are an organism walking down the street, and you are immune to the rare virus squirrelpox. A squirrelpox virus is walking on the same sidewalk towards you. It sees you, and goes, “that one’s immune to me,” and quickly crosses the street, going nowhere near you. Beause you are immune.

This is what immunity often is: You have built up an immunity to a common cold virus. Somebody infected with that virus sneezes on you and now that virus is in you. It begins to reproduce and do its thing, and you develop cold symptoms. However, your adaptive immune system has seen this virus before, so it quickly mounts a defense, so even though you do get a cold, you fight it off quickly and in five days you feel fine.

Lots of times, though, immunity works like this: You have an immunity to a certain disease. Perhaps you had that disease earlier in your life and your adaptive immune system developed a strategy to attack this pathogen next time it comes around. Perhaps you got a vaccine that prompted your adaptive immune system to develop a strategy to attack this pathogen next time it comes around. The virus goes in you — the virus does in fact infect you, it does not “cross the street” to avoid you. But your body is so ready for it that the counter attack is fast and effective, and before you can either develop symptoms or start passing the disease on to someone else, your body’s immune system has literally killed it.

An acquired or induced immunity can be called “100%” and it can be “life long” but it is never able to actually keep the disease out, and it is likely that few, if any, adaptive immune system build-ups last for the entire life of a person who lives a long time. Some immunity does not stop you from getting sick but does cause you to get better faster, and some immunity doesn’t last that long.

Much of the misunderstanding about immunity comes from the fact that our understanding of immunity comes from two distinct diseases: Polio and influenza. Polio vaccine is famous because its invention and deployment was historic and significant. Polio vaccine confers a strong immunity, one that is seen as life long and complete. Even this is not so simple, but if you believe what I just said about polio vaccination and immunity you would be in the ballpark. Influenza immunity is often discussed because it is at the center of the anti-vax debate, everyone gets the flu now and then (or so it seems) and the so-called “flu vaccine” is supposedly only “60% effective” or thereabouts, and thus, being imperfect, is the focus of rage on the internet as though it was a candidate for office.

If polio is an outlaw gunslinger in the old west, and the polio vaccine is Marshal Dillon, then influenza is all the underground crime organizations imagined in fiction and the flu vaccine is a competent but underfunded police agency.

When we say that the influenza vaccine is 50% effective in a give year in the US, as an example, what that can mean is that there are five kinds of flu circulating at various proportions in the population, and there are three kinds of vaccine in the shot you get; maybe two of those vaccines are nearly 100% effective in immunizing a person against two of the circulating influenza viruses, one of the viruses is untouched by the vaccine but doesn’t get you that sick, and one of the vaccines is for a virus that never really showed up, and the leftover viruses are the ones doing most of the damage. Or something along those lines. The outcome is, across the population, that the average vaccinated person in the population under consideration would have their chance of getting the flu if exposed is half what it would have been were they not vaccinated. So, 50% effective that year. Some other year these parameters may be very different, and the “vaccine” (a mix of different vaccines in one shot) is different. And, each vaccine may itself have a higher or lower level of effectiveness.

And that is the simple version of the story.

Immunity is not a folk concept. It is a medical concept. The fact that many people believe that immunity is the inability of a disease to affect a person, which is 100% wrong in every way, is not relevant to anything but people’s misunderstanding of the concept.

When we hear that there is a certain possible reinfection rate of COVID-19 in China or Japan, this does not mean that people don’t get immunity once they have the disease, or that COVID-19 has special powers. One health expert misstated that since we don’t know for sure what acquired immunity to COVID-19 looks like, we can’t assume that it is long term. That is balderdash. It is very likely long term (if “long” is years) because that is what normally happens. This statement is like looking at the first new car off the line of a new make and model and saying, “since we’ve never actually seen one of the drive, we have to assume there is a good chance none of these cars will work.” There may be a few recalls in the future of this make and model car, but it will work.

We can assume normalcy, we can assume biology to do what biology does. Bill O’Reilly does not know how tides work, but someone else does. Normally, adaptive immunity occurs, and lasts for a good time. Normally, immunity to certain kinds of viruses can be less than 100%, so there is some getting sick, and normally, a subset of people don’t develop much of an immunity because their own immune system simply fails at that task. COVID-19 will ultimately be found to match normal biological expectations, though we don’t know the details yet, and we won’t for some time. The fact that normal biological expectations do not form the basis of folk thinking about this disease, or pathogens and immunity in general, does not make Covid-19 a preternatural force, or an unknowable thing.

Still, remain hiding in your house until the all clear.

There is another level of thinking about immunity that I won’t go into detail about right now, but I’ll mention. We often, rightly, think of immunity at the population level, even though it does, truly, work at the individual and molecular level. Assume a particular vaccine, or exposure, typically provides ~100%) immunity in individuals. If 10% of the population have that immunity at the start, the disease will act like nobody is immune, as far as we’d be able to see. Often, natural (genetic?) immunity at low levels exist in a population, and can only be discovered by intensive research over a long time. If, on the other hand, 90% of the people in a population are ~100% immune, the disease may be so unable to get a foothold that it is like it isn’t there. The point is, the appearance of a diseases behavior seems to range from 0% (there ain’t none) to 100% (it’s everywhere!) on the surface, but this outcome is a function of a much smaller range of actual immunity values, like the 10-90% just noted, or more likely, closer to 0-70%. Putting this another way, a population gets very close to “immune” at the population level as the proportion of individuals who can’t get and pass on the pathogen rises over about half. This is called herd immunity. It will take several cycles of COVID-19 infection to achieve natural herd immunity, most likely, unless a vaccine is found. But once that happens, the disease is likely to stay around at low levels then occasionally come back and be menacing, but not as bad as it is now, on occasion.

So, let’s get that vaccine going!