genetic preparedness for both. The upside is that a vector-borne virus has a vehicle that can carry it some distance, searching thirstily for new hosts. A sneeze travels downwind, more or less at random, but a mosquito can fly upwind toward a victim. That’s what makes vectors such effective modes of transmission.
Blood-borne viruses can also spread to new hosts by way of hypodermic needles and transfusions. But those opportunities are adventitious addenda, recent and accidental, patched onto ancient viral strategies shaped by evolution. Ebola and HIV-1, two viruses of very different character, very different adaptive strategies, both happen to move well via needles. So does hepatitis C virus.
In the case of Ebola, transmission from human to human occurs also by blood-to-blood contact in intimate situations, as when one person takes care of another. For a nursing sister in a Congolese clinic with small cracks on her chapped hands, a few minutes spent wiping bloody diarrhea off the floor could be exposure enough. This is extraordinary transmission, so far as the virus is concerned. Ordinary transmission is however Ebola gets from one individual to another within whatever animal host—identity still unknown—serves as its reservoir. Ordinary transmission allows the virus to perpetuate itself. Extraordinary transmission gives it a burst of high replication, high notoriety, but soon brings it to a dead end. Passing between people via bloody rags or reused needles, in this or that African clinic, is not a strategy that serves Ebola for long-term survival. It’s just an occasional anomaly that has little or no significance (so far, anyway) within Ebola’s broader evolutionary history. Of course, that could change.
Ordinary transmission, for Ebola, need not be blood-borne. If the virus resides in fruit bats of the Central African forest, as suspected but not yet proven, then it might pass from bat to bat during sex, or suckling of infants, or mutual grooming among adults, or breathing on one another, or biting and scratching, or any other form of close contact. At this point in Ebola research, we can only guess. Drops of urine, falling from one bat into the eyes of another? Saliva on shared fruit? Blood-sucking bat bugs? Saliva on fruit would explain how Ebola gets into chimpanzees and gorillas. Bat bugs (yes, there are such things, related to bedbugs) would allow us to imagine a specialist parasite I’ll call Cimex ebolaensis. It’s all speculation. We might even come to learn that Ebola is a natural infection of African ticks, who carry it among fruit bats, gorillas, and chimps. Merely a thought. But please remember that I’ve just invented tick-borne Ebola from zero evidence.
Sexual transmission is a good scheme for viruses with low hardiness in the external environment. It’s a mode of passage that doesn’t require them to go outside. They’re virtually never exposed to daylight or dry air. The virions pass from one body to another by way of direct, intimate contact between host cells lining delicate genital and mucosal surfaces. Rubbing (not just pressing) those surfaces together probably helps. Transmission during coitus is a conservative strategy, reducing risk to such viruses, sparing the need for hardening against desiccation or sunlight. But it has a downside too—notably, that opportunities for transmission are fewer. Even the most lubricious humans don’t have sex as often as they exhale. So the sexually transmitted viruses tend toward patience. They cause persistent infections and endure long periods of latency, punctuated by recurrent outbreaks (like herpesviruses); or else they replicate slowly (like HIV-1 and hepatitis B) up to a critical point at which things get bad. Such patience within a host gives the virus more time and therefore more sexual encounters by which to get itself passed along.
Vertical transmission, meaning mother-to-offspring, is another slow, cautious mode. It can occur during pregnancy, during birth, or (in the case of mammals) by way of milk while an infant nurses. HIV-1, for instance, can be transmitted from mother to fetus across the placenta, or to a newborn in the birth canal, or through breastfeeding; but each of those outcomes is far from inevitable, and the likelihood of their occurrence can be lowered with medical precautions. Rubella (loosely known as German measles) is caused by a virus capable of vertical as well as airborne transmission, and it can kill a fetus or inflict severe damage, including heart disorders, blindness, and deafness. That’s why young girls were counseled, in the era before rubella vaccine, to get themselves infected with the virus—suffer a mild bout and be done with it, permanently