Why the common cold is more successful than its exotic cousins

HIV? Avian Flu? Amateurs! The real winner of evolution is the common cold.

The virus is evolution incarnate. Little more than a strand of DNA or RNA coiled inside an insidious delivery capsule, it is as close to a complete reduction of biology as can be. It contains the heart of life, the genetic code, and only the barest of bodies.

Among viruses there are many that evoke wild places of the world, with wild illnesses springing from them: Yellow fever, Dengue, Ebola. The names conjure fear. They also evoke imagery of rainforests, isolated treks into equatorial wilderness, and exotic viruses living in exotic places.

Our perspective as the environment in which viruses thrive–and all the unpleasantness they bring–is very different from the perspective of the virus itself. If a virus is simply a genome that evolves, then it’s success is measured by its survival and proliferation. In that sense, the winner, the greatest virus of them all, is none other than the rhinovirus, which causes the common cold.

“Somewhere on the order of 5 percent [of humans] are infected at any one time and each one of these has trillions of viruses,” said Fred Adler, professor of mathematics and biology at the University of Utah. To investigate the astonishing success of the rhinovirus, he and a small team of researchers devised the hypothesis that the rapid pace of viral evolution contributed to the number and variations of rhinovirus serotypes that infect human populations at any given time.

Adler applies mathematics to gain greater insights into biological systems, which doesn’t always work. “We’re almost always wrong,” he said. But when his team examined the literature on the common cold, it appeared to them that the rhinovirus evolved faster than other viruses.

Using 3-dimensional computer modeling, Adler and his colleagues tested their hypothesis by recreating the ever-evolving structure of the virus capsid. The amino acids present on exposed regions of the capsid, which bind to cell surface proteins and enable the virus to infect individual cells, are constantly changing. Adler thinks this is part of the reason why the rhinovirus can penetrate populations so effectively.

The team extracted data from the Seattle Virus Watch study, a large investigation conducted in the 1960s on the link between rhinovirus subtypes and the common cold. From this data, they classified multiple serotypes of rhinovirus and applied that information to their observations of molecular evolution.

They discovered that each serotype evolved at a different pace, conferring greater fitness on those able to adapt faster. This also explained the greater penetration of certain subgroups of the virus in different populations around the world.

While Adler set out to better understand the viral mechanisms underlying the common cold, they ended up finding correlations with more practical purposes. “There are clinical applications,” he said, “And asthma is the most important of them.” The wealth of information from the previous studies combined with modelling of the viral structure paved the way for the researchers to see causal links between certain rhinovirus serotypes and the occurrence of asthma in certain groups of people.

While the exotic names of wild diseases still inspire fear whenever there’s a distant outbreak, the most common of ailments continues to hold Adler and his colleagues in thrall. On the tail of this most recent publication, the team followed up with a paper modeling the immunology of asthma. By combining the epidemiological data with new findings on rhinovirus serotypes and external variables such as stress and pollution, Adler believes the team can help understand more about this common and perhaps underreported health issue. And while such common maladies may not be as interesting as rare hemorrhagic fevers, to Adler, the rhinovirus–and particularly its relationship to asthma–is “a deeply interesting issue.”