Who could forget the climax of H. G. Wells' The War of the Worlds? Just when the whole of humanity lies prostrate under the destructive weapons of the Martians, the survivors emerge to discover that all the invaders are dead or dying, all at the same time, having no resistance to terrestrial germs, which they all caught simultaneously, and destroyed them all within the same time span. It makes a dramatic picture - until you start to think about it.
Germs aren't just floating around, ready to pounce on any unprotected organism. Even if the victim is immuno-compromised, the germs which attack it are only those that can thrive in that species. Germs are specific to their hosts and the body part of their hosts.
Take the malarial Plasmodium. It is injected into the blood stream by an Anopheles mosquito (any old mosquito won't do), and migrates to the liver, where it proliferates asexually before re-entering the blood stream, where it feeds on the red blood corpuscles. They burst, and then it spreads to other corpuscles. Eventually, it gets picked up by another Anopheles mosquito. That is a simplified version of its life cycle, of course. Each stage of the life cycle requires a particular environment. It can only live in the liver and the red blood cells. It cannot live in the lungs or gut, assuming it could ever get there. And it is limited to human beings and some close relatives. Inside a cow simply will not do. So how on earth (or anywhere else for that matter) could it survive in the liver of a Martian (assuming that they have livers), or a Martian's blood stream - which presumably functions differently to ours?
All right, malaria has a complicated life cycle. Let's try our favourite gut bacterium, E. coli. It lives, multiplies, and gains its nutriments in the lower bowels of warm-blooded animals, such as us. It is transmitted by water contaminated by faeces, but it cannot live in the mouth, pharynx, gullet, or stomach, although it must pass through them in order to reach the bowels.
Parasites are host-specific. Some are extremely specific. Smallpox, for example, was completely restricted to human beings. Others are more catholic in their choice of hosts. Indeed, many of our classic epidemic diseases are believed to have evolved from parasites endemic to our livestock. Both humans, cattle, and similar animals can get sleeping sickness. Pigs, ducks, and humans, among others, can suffer from influenza. However, even then there are restrictions. I have never heard of a dog catching sleeping sickness, or a reptile getting the flu. It is not just a case of their immune system repelling them; their internal environment is wrong.
And while we are on the subject, in 2011 there was a (deservedly) short-lived TV series called Terra Nova, about time travellers colonising the earth of 85 million years ago. In one episode, a colonist was found to have an enormous worm in his intestine. Impossible! No worm evolved to live inside dinosaurs could thrive inside a human being. The only diseases which colonists of other planets or other times could suffer from would be the ones they brought with them. If they were wise, they would practise a very strict quarantine.
Viruses are a special case. Probably most of you are aware that our genetic code is contained in DNA, and that it works by manufacturing proteins with the help of RNA. Viruses cannot reproduce by themselves. They consist simply of a string of DNA or RNA wrapped in a package of protein and lipid. They work by latching onto a host's cell by connecting with its chemical signature. That is why only certain hosts are possible. The virus then injects its DNA/RNA into the host cell, and the host cells manufactures the virus for it until it (the cell) bursts. But this assumes that the virus and the host possess the same genetic code - which they must, because they both evolved from the same original life form. But, as explained in my article on interplanetary hybrids, aliens will almost certainly be operating on a different genetic code.
Moral of the story: alien life forms would be immune to our infectious diseases. A human visitor to another planet would be immune to its plagues, no matter how severe. However, any wounds or constitutional diseases, like high blood pressure or arthritis, suffered by a space traveller could not be treated with any alien medications, for his physiology would be different.
On the other hand, certain writers such as John Norman and Alan Burt Akers have created planetary romances set on worlds which have been seeded with human life by more advanced aliens. If a visitor from earth were to arrive, remember what happened down here. When Europeans arrived in the Americas, their epidemic diseases swept through the native populations are reduced their numbers by up to 90% in the first century. On the other hand, when the same Europeans arrived in West Africa, the horrible endemic diseases turned the place into "the white man's grave". At the very least, the visitor should come down with diarrhoea just like modern tourists.
My next article will discuss the strange artwork involved in four-armed humanoids.
But you can return to the Index here.