“A hen is only an egg’s way of making another egg.”

- Samuel Butler

Highly pathogenic avian influenza

A virus is a set of instructions to make the proteins that allow it to spread and reproduce. Viruses have no legs, no wings, no way to move around. They even lack the whip-like appendages that many bacteria use for locomotion. So viruses must trick the host into doing the spreading for them. One sees similar situations throughout nature. Plants can’t walk from place to place, so many have evolved flowers with sweet nectar to attract bees who spread the plants’ reproductive pollen for them. Cockleburs have barbs to hitch rides on furry animals; berries developed sweetness so that birds would excrete seeds miles away. Viruses represent this evolutionary instinct boiled down to its essence.

The rabies virus, for example, is programmed to infect parts of the animal brain that induce uncontrollable rage, while at the same time replicating in the salivary glands to spread itself best through the provoked frenzy of biting.¹⁵² Toxoplasma, though not a virus, uses a similar mechanism to spread. The disease infects the intestines of cats, is excreted in the feces, and is then picked up by an intermediate host—like a rat or mouse—who is eaten by another cat to complete the cycle. To facilitate its spread, the toxoplasma parasite worms its way into the rodent’s brain and actually alters the rodent’s behavior, amazingly turning the animal’s natural anti-predator aversion to cats into an imprudent attraction.¹⁵³

Diseases like cholera and rotavirus spread through feces, so, not surprisingly, they cause explosive diarrhea. Ebola is spread by blood, so it makes you bleed. Blood-borne travel is not very efficient, though. Neither is dog or even mosquito saliva, at least not for a virus that sets its sights high. Viruses that “figure out” how to travel the respiratory route, or the venereal route, have the potential to infect millions. Of the two, though, it’s easier to practice safe sex than it is to stop breathing.