The recent measles outbreak has everyone taking another look at "childhood" diseases. Why are these diseases, relatively mild in children over five, so often devastating to adults? An infectious disease expert talks to io9 about possible explanations for why these viruses hit us hard when we should be at our healthiest.

Top image: Associated Press

Measles, Mumps, and the U-Shaped Curve

The recent resurgence of measles has a lot of people nervous. They're not the people who would usually be worried about an outbreak of disease. Strong, healthy adults are looking twice at the people they sit next to in doctors' offices and on trains. And with good reason: These adults are more likely than children to suffer complications, ranging from pneumonia to inflammation of the brain to damaged corneas.

In fact, there are a host of diseases, commonly known as "childhood" diseases, that, while not a picnic at any time of life, are relatively gentle to older children, but unkind to anyone out of adolescence. While most sites and papers note the various ways that these diseases are more likely to ravage adults, few mention any reason for why adults are so vulnerable.

I asked Dr. John Swartzberg, a clinical professor at the Berkeley School of Public Health, a few questions about childhood diseases. To begin with, he confirmed that, yes, "One of the factors that determines the severity of the illness is the age of the patient."

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While any single patient can respond well or badly to a given illness, patterns based on age do emerge. If we were to look at a chart showing the mortality rate of, for example, the typical influenza virus as a function of age, we would be looking a a U-shaped curve. It would be a very wide U, with high mortality rates for the very young and the very old, while the mortality rates for everyone in their middle years would be nice and low.

This is understandable. A very young child's immune system is still getting set up, and the older we get, the less well the immune system responds. The mortality rate drops for healthy adults because their immune system is doing its job.

Childhood Diseases and the Skinny U

But diseases like measles give us charts that don't look much like the big, comforting U that influenza has. Measles is extremely dangerous to children under five, and dangerous to adults — even ones in their early twenties. The U-shaped curve for measles is a skinny dip. that leaves most of us in danger, and only older children and very young adolescents are in the region with low mortality rates.

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Some papers speculate that, "The increased severity of measles in adults most likely reflects the decline in cell-mediated immunity that begins in adulthood." Cell-mediated immunity employs not antibodies, but big, hungry cells called phagocytes, which eat any foreign material in the body. This idea is, for now, only speculation.

When I press Dr. Swartzberg about why childhood disease hit adults so hard, he emphasizes that he, too, is speculating. He believes that these viruses and humans could be examples of "host-parasite interaction," and that we have "adapted to each other," over long periods of time. Those adaptations are dependent on us contracting diseases at a specific time of life.

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This isn't as odd as it sounds. Although we think of our bodies as fighting invading viruses, the relationship isn't adversarial. The measles virus isn't "trying" to kill us any more than the polio virus was trying to kill us.

The polio virus came to be known as a fearsome killer of children. That, according to Swartzberg, was because no one understood the virus's own, very skinny, U-shaped curve. Polio, when contracted by very young children, isn't nearly the killer we think it is. The polio virus is spread through infected fecal matter. When the public water supply consisted of rivers, lakes, wells, and pumps, infected fecal matter and drinking water mixed regularly. Infants were exposed to the virus early. When the government cleaned up drinking water — saving infants and adults from many other diseases — young children no longer came into contact with the polio virus. It was only later in childhood, when they went swimming in pools and streams, that kids contracted the virus. The parasite and the host no longer had matching adaptations. The disease that, for the most part, had been mild, became devastating.

That being said, even a disease that is "for the most part" mild can have terrible consequences, and there's no comfort in being in the shallow part of the U, if you're below the mortality line. In 2013, there were 145,700 deaths due to the measles. Before the measles vaccine became widely available, there were 2.6 million measles deaths per year. Some of those deaths were of children who got measles at the "right" age, when the effects of the disease were supposed to be mild. Some of those deaths were of people at the wrong age, who caught measles from children at the right age.

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In short, there is no "right" age for measles, or chickenpox, or any other childhood disease. There's a right age for vaccination, and we are living in it. If you want the best chance at getting through life with your brain the right size and your corneas intact, get the shot.

Many thanks to Dr. John Swartzberg for his time and expertise.

Measles Sufferer Image: Wellcome Images, Phagocyte Image: PLOS

[Other Sources: World Health Organization, The Clinical Significance of Measles: A Review]

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