Why viruses are hard to kill
Viruses are among the biggest threats to humanity, with the current pandemic showing how these pathogens can shut down countries, halt entire industries and cause untold human suffering as they spread through communities.
Viruses have also evolved in such a way that they are difficult to kill. What makes them, including the coronavirus, so tricky to cure?
Part of the problem is the nature of viruses themselves. They exist like freeloading zombies — not quite dead, yet certainly not alive.
"Viruses don't really do anything — they're effectively inert until they come into contact with a host cell," said Derek Gatherer, a virologist at Lancaster University in the United Kingdom. "But as soon as that happens, they switch on and come to life."
The odd makeup of these infectious agents is part of what makes them difficult to defeat. Compared to other pathogens, such as bacteria, viruses are minuscule. And because they have none of the hallmarks of living things — a metabolism or the ability to reproduce on their own, for example — they are harder to target with drugs.
"The fact that they are not alive means they don't have to play by the same rules that living things play by," said Britt Glaunsinger, a virologist at the University of California, Berkeley.
But a virus can't break into just any cell in the body. Instead, one of its proteins will bind to another protein — akin to a key fitting into a lock — which then allows the virus to hijack certain cells. With this outbreak, the coronavirus' so-called spike protein primarily fits "locks" that are present on lung cells, which is why COVID-19, the disease it causes, is mainly a respiratory illness.
Once the invasion takes place, the cell in essence is transformed into a factory that churns out hundreds and hundreds of copies of the virus, based on instructions encoded in its genetic material — RNA, or ribonucleic acid, in the case of the coronavirus.
"It basically acts like a thief inside the cell, stealing all of the cellular machinery and repurposing those machines to make more of the virus," Glaunsinger said.
The human body has evolved defense systems to protect against these kinds of infections.
First, cells have a built-in alarm system to detect viral invaders. The presence of an intruder triggers what's known as an innate immune response, which can involve the host cell releasing a protein that tries to interfere with the virus' replication or can involve the immune system trying to shut down the compromised cells.
But sometimes, these defense mechanisms aren't enough.
The work of these reinforcements to try to defeat the virus is typically what causes the symptoms of a viral infection — in other words, it's at this point when a person may come down with a fever and start to feel sick.
When the immune system is finally triggered, it can also kick into overdrive, causing what's called a cytokine storm, which is thought to be the root of some of the most severe coronavirus cases.
"There's a lot of data coming out that some of the damage might be due to a very strong and brisk immune response, where the body is fighting back and sort of throwing everything it has at the virus," said Dr. Adam Lauring, an associate professor of microbiology and immunology at the University of Michigan in Ann Arbor. "While that may control the virus, it also causes a lot of damage to the lungs."
Journal of Infectious Diseases and Diagnosis