Leprosy is not a disease of the past. Nor is it particularly rare. It’s one of the many diseases we get without ever knowing we have them . . . most of the time.

The Viruses

Diseases don’t always make their presence known. This is because, although we talk about “fighting” a disease, we’re the only ones in the fight. A virus has no interest in making us miserable. In some case, viruses have a better survival rate if they don’t give their hosts any trouble.

One virus in particular spreads more easily because people don’t know they’re infected. At most, one quarter of people who have herpes simplex virus type 2 have any idea that they carry the virus. Only a small percentage of people have regular outbreaks. Symptomless carriers often never realize that they’re infected. Despite their lack of outbreaks, they can pass the virus on to their next partner. Though they actively shed the virus about half the time that people with symptoms do, they can be more likely to pass along their infection, simply because they don’t know they have it.

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There’s more than one way of “not knowing” whether you have a disease. Most of us at some point during our winters spend time wondering if we have the flu or if we just ate a bad shrimp cocktail. Diseases can be so mild that they masquerade as minor ailments. Perhaps the strangest one of these is polio. Polio rose to prominence in the late 19th and early 20th century. Every summer, people were terrified when outbreaks hit cities, seemingly without reason. While there are some people who can carry the virus while remaining seemingly healthy, there are others who regularly carry it while seeming to have nothing more than an unpleasant flu. The virus spreads when infected fecal matter gets into the water supply, so before water treatment babies caught the virus very young. Although it could be severe, for the most part it was just another “childhood disease” that usually ran its course without incident. As water safety improved, children stopped being exposed to the virus as infants and toddlers, and started being exposed at school, or when they went swimming in pools or lakes during the summer. It was then that just another anonymous childhood bug became an infamous, and life-threatening disease.

The Bacteria

It’s nearly impossible to estimate the people who, temporarily, get infected with leprosy. The fact that leprosy was so prevalent that entire islands had to be devoted to the people who had it gives us the wrong impression of the virulence of the disease. We come into contact with leprosy when an infected person coughs or sneezes on us and the bacteria Mycobacterium leprae gets into our system. We realize that we’ve been infected with the disease . . . never. Ninety-five percent of the time, our own immune system eliminates the bacteria and we never show symptoms.

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Those whose immune systems can’t kill off the bacterium can look forward to up to twenty years of absolutely nothing happening. This is one of the reasons why leprosy has lasted so long. An infected person can spend a very long time showing no symptoms, and spreading the bacteria around. There are also animals that are likely to carry leprosy — armadillos are repositories for the disease, because their low body temperature provides the best environment for the bacterium. If you do come down with leprosy, never fear. Leprosy is curable with a multi-drug cocktail, so you don’t have to hope for a miracle.

Legionnaire’s disease is another very rare response to a very common infection. We’ve probably all inhaled legionella bacteria. A few of us have even gotten sick with a rare, flu-like version of the disease known as Pontiac fever. But the bacterium usually can’t do much damage until it gets into the system of someone who is older, has a compromised immune system, and is a heavy smoker.

More worrying is tuberculosis. The bacterium responsible for tuberculosis, Mycobacterium tuberculosis, is more infectious than the bacteria that causes leprosy, and there are rare drug resistant strains that cause death between thirty-five and fifty percent of the time. Ninety percent of the time, however, a person infected with tuberculosis has no idea they have it. It’s another scary disease that doesn’t even show up in most of the people that have it.

But the fact that people don’t consciously know they are infected doesn’t mean their immune system isn’t fighting the infection. When the immune system is weakened, the infection becomes serious, or deadly. This is why tuberculosis is such a killer in populations without access to medical treatment. When one treatable, or at least survivable, illness sweeps through the population, it can set off a wave of deaths from tuberculosis.

One strange quirk of the immune system can lead to bacterial infection “seasons” — times during which both leprosy and tuberculosis flare up in a population. Unsurprisingly, bacterial infections become serious after flu season. The virus taxes the immune system and leaves it vulnerable. But there’s evidence it does something more specific. Interferons are signaling proteins which let the immune system know the kind of response it needs to make. There are signaling proteins that are specific to viruses, and signaling proteins which are specific to bacteria. Not only do interferons specific to viruses not help fight bacteria, but they can actually suppress the immune response to bacteria. So when a seasonal virus spreads through a population, it can trigger a correspondingly seasonal bacteria.

And since, technically, both the virus and the bacteria can be in your system without you knowing it, a health problem can spring up, seemingly from nowhere.

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Top Image: NIAID, Herpes Image: George W Beran, Leprosy Bacterium: Public Health Library

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