It is always a shock, isn’t it? When two life forms make contact which until then had been totally separated?
The viruses, when they struck, nearly wiped out the primitive Martians.
Nearly, but not totally. Total eradication was neither in their power or in their interest. Interest because the newly-born strains depended on their newly-found preys to survive: viruses infect other cells in order to reproduce. They are very simple creatures, at the border between life and non-life, and they lack the complex machinery others – whether amoebae, slime molds or the specialised component of a multicellular organism – use to duplicate themselves. So they have to hijack these mechanisms and trick them into making copies of their genetic code instead of the host’s own DNA. After a while the mass of freshly assembled viruses become too numerous and too disruptive and the cell dies but they cannot afford to be too efficient: finding a new host is a complex and hazardous process and those who kill their victims too fast cannot propagate. The more virulent are the first to die out.
They were never anyway that virulent in the first place. This was after all the first time they came into contact with multicellular life and all its quirks, and in consequence their mode of attack was never that complex. Slowly, generation after generation, the proto-surviving Martians were able to adapt, develop defences against these minute invaders. The process was not that different from what happened on Earth in that the potential hosts were mainly concerned with disrupting their attackers replicating strategy: either by attacking them inside the cells they were using as factories, or by destroying the compromised cells altogether.
As on Earth, a kind of arm-race took place, but it never resulted in the kind of Mexican stand-off we are used to here: on Mars the viral threat was nearly eradicated.
Nearly. That word again.
Human scientists would have called them a neurotropic disease. The proto-Martians were still far away from having scientists, they were far away from having languages or even a proper thought process. But they had enough of a nervous system to die in great numbers and agony. This new strain carefully avoided the places where their host immune system was at its most concentrated: the circulatory system and the breathing apparatus. Instead they attacked the nerves the one place where the ancestors of the Huylee were nearly defenceless.
It is a hard job, living in nerve cells, and a neurotropic virus is a slow creature. Unable to jump into the ichor stream to disseminate itself, it had to crawl from cell to cell, neuron to neuron, slowly destroying its host’s nervous system until it could no longer move, or feed, or breathe. After its death, huge quantities of the virus would be released into the wider world, to find their way back into the slime, to wait there and eventually to be grazed upon by their next victim.
Clearly, something had to be done, a new answer had to be found. But try explaining to evolution that sometimes the cure is worse than the disease.