The Virus Story

The term “virus” comes from Latin for “poison, sap of plants, slimy liquid”. A non-living thing, in other words, writes David Quammen in Spillover. Evidence to the contrary came not from medicine but from agronomy!

 

The tobacco mosaic disease had hit tobacco production. It was transferrable via the sap. Filter the sap using filters through which bacteria can’t pass. No effect. The filtered sap could still spread the disease. This thing was tinier than bacteria.

 

A Dutch researcher, Martinus Beijerinck diluted the sap, and then infected another plant even with that diluted dose. Weirdly:

“Whatever it was, (it) regained its full strength even after dilution.”

That meant it was “reproducing itself”. No poison can do that. No by-product of a bacteria can do that. This was something else.

 

Trial and error yielded new information:

“In a container of filtered sap alone, it wouldn’t grow.”

Which meant it wasn’t feeding on the sap:

“It needed something else. It needed the plant.”

 

The last point, its need for living cells, also made it impossible to cultivate in lab cultures for a long time. Which in turn made it hard to study. But everything we knew then were still clues, as we would come to learn later:

“Its size is small and so its genome, simplified down to the bare necessities for an opportunistic, dependent existence. It doesn’t contain its own reproductive machinery.”

And yet, the virus has ways to (1) Get from one host to another, (2) penetrate a cell within that host, (3) use the host cell’s machinery to make copies of itself, and (4) get back out.

 

A virus’s genetic code can be DNA or RNA. DNA, of course, is famously double-stranded, making it possible to detect and correct errors in copying. RNA, with its single strand, can’t detect or fix errors in copying. Which of course means that error rates in RNA viruses are higher. Put in different terms, the mutation rates of RNA viruses are way higher. Obviously, many of those errors are fatal. Then again, several of those errors are beneficial (to the virus).

 

For example, consider mutations whereby a virus can survive in two different species. If one of those can serve as the vector (carrier), that virus has hit the jackpot. After all:

“A sneeze travels downwind, more or less at random, but a mosquito can fly upwind toward a victim.”

 

Viruses are so fascinating. And yes, deadly at times.