I got mononucleosis when I was 15 or so and was reading up on it recently because why not. The Wikipedia article says the Epstein-Barr virus *usually* causes it but that other viruses can do as well. How would two different viruses cause one disease? Shouldn’t they be two different diseases even if they have similar symptoms?
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What we perceive as the disease is often the signs /symptoms caused by the body’s immune response in action
For example, pneumonia is the lungs filling with fluid and keeping a patient from breathing. We can see the fluid in an x ray.
But what caused the fluid? Bacteria? Virus? Other injury or pathogens? They’re all pneumonia and lungs with fluid.
Historically, when people refer to diseases, often what they’re referring to isn’t a single cause, but rather a bundle of symptoms that all come together. Consider for example things like “the flu”, “the common cold”, “pneumonia”, or “food poisoning”.
This is especially the case because we didn’t always know what caused diseases. Figuring out exactly what infectious agent is causing an illness requires labwork, and can sometimes require very expensive equipment – for example, you probably can’t tell which virus someone has without PCR or electron microscopes.
Mono it turns out is a disease mostly of the throat caused by a specific virus…but also, sometimes other viruses can cause almost identical symptoms. In almost all cases, the treatment is the same (that is, rest, fluids, monitoring, NSAIDs for the fever and pain). So there’s not really any point in distinguishing between Epstein-Barr mono and CMV mono for the vast majority of cases. If it got bad enough that you had to be hospitalized and they were considering using antivirals, they might check to see what strain you had, but short of that it doesn’t really matter.
Mononucleosis is “a syndrome that resemble[s] an acute infectious disease accompanied by atypical large peripheral blood lymphocytes” ([source](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4670567/)).
Let’s unpack that.
A lymphocyte is a type of white blood cell. White blood cells are the blood cells of the immune system. If you have mononucleosis, many of your lymphocytes kind of “swell up”, becoming atypically large.
So why the name *mononucleosis*? Well, lymphocytes happen to be *mononucleotic*. This refers to the shape of the nucleus of these cells. The nucleus is a structure inside the cell that holds the DNA. A typical nucleus just looks like a nice round ball. But in some white blood cells, it looks different – not all nice and round but more odd-shaped with multiple lobes. Some white blood cells also seem to be filled with little granules, almost like they have multiple nuclei. The point of a *mononucleotic* white blood cell is that it doesn’t have any of that stuff going on. It just has the “single nice round ball”. So that’s how we get the *mononucle-* part of the disease’s name: *mono* meaning “single” and *nucle* coming from “nucleus”.
That leaves the *-osis* part, which means something like “abnormal condition”. So, now we understand the name *mononucleosis*: it just means that a particular type of white blood cell is in abnormal condition. Specifically, the lymphocytes are enlarged, as we mentioned earlier.
So, the name “mononucleosis” can be applied to any disease that causes this syndrome of having these enlarged lymphocytes. It doesn’t matter what pathogen causes that.
The reason that multiple viruses, mainly the Epstein-Barr virus (EBV) and Cytomegalovirus (CMV), can cause this same syndrome, is because these viruses infect the lymphocytes themselves. Most viruses don’t infect white blood cells, but EBV and CMV both do. The infection and the body’s response to it cause the lymphocytes to swell up to an abnormal size.
Many of the other symptoms of mono are a direct or indirect result of these swollen lymphocytes, so they are also very similar between different viruses.
Mononucleosis is a symptom, not a disease. Just like a fever or cough, it could have a number of different underlying causes. It’s the result of your immune system responding to something that isn’t supposed to be there. A disease is a collection of symptoms that have a known pattern or cause.
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