For a short story I'm writing for no reason whatsoever, I have a planet full of human looking aliens. Only thing different between humans and these aliens is that the aliens die if they sneeze. What changes do I need to make for this to be possible? Would it have an outward appearance, or do I just change the organs around?
The aliens went through a pandemic at some point in their history. Lacking the science to understand it, they instead enshrined in their religions that people who sneeze are sinners who must be killed on sight.
To make it more complicated for the aliens who sneeze, their anatomy is such that sneezing blows open a flap of skin on their noses. It either takes too long to regenerate, or it does not regenerate at all. Plastic surgery won't close it perfectly either.
Maybe they just got canary lungs, which collapse if they sneeze because that puts a lot of pressure inside.
It can be that sneezing has been phased out, for whatever reason. Somehow they just don't sneeze anymore under their current circumstances. After many generations, the reflex might still be in place, like nipples on a man. Besides being there, in rare cases they are able to give off a liquid, suggesting that reflexes, despite being useless, can be maintained for a long period. The reflex might still be activated under rare circumstances.
After so many generations, they might've developed the folowing:
- A sneeze is incredibly powerful. The bones of the ribcage or the ligaments might've weakened over time due to the more protected lifestyle. The bones could shatter or the ligaments torn from the ribcage, rendering them impossible to breathe. It isn't instant and with assistance from bystanders they might live until they can get a breathing machine. This also has heavy implications for their lifestyle for generations, as many sports could also break or injure them gravely. So they probably don't do these.
- Due to the foreign nature of the sneeze, they might try to keep it inside. The pressure of a sneeze is quite inpressive. This pressure could damage the skin and flesh between the brain and the mouth/nose, putting them at risk of infection or just the pressure on the brain is too much, crushing vital parts. The brain can actually survive quite a lot, see whiplash and concussions. If you're doing death by sneezing anyway it could start an epileptic episode that kills them. Or maybe all pressure escapes through a small patch of skin where a loose bone lays beyond, pushing it straight through the brain.
- The reflex of a sneeze might leak into other reflexes after all this time. The "bleeding" of feelings and such mostly happen in the brain. As as example, amputee persons might feel an itch in their amputated limb, yet scratching some place on the body can solve this. Reflexes might "bleed" into other nerves as well, as they are controlled mostly in the spine. The relex bleeding into the nervous vagus can cramp the heart, making it unanimously contract with dangerous results.
- After all this time it'll have an hormonal response that will eventually kill the alien.
Each of these is is very implausible, but you van give it a shot.
A Sneeze is the Alien’s Banshee Cry
A sneeze, or sternutation, is a semi-autonomous, convulsive expulsion of air from the lungs through the nose and mouth, usually caused by foreign particles irritating the nasal mucosa.
The function of sneezing is to expel mucus containing foreign particles or irritants and cleanse the nasal cavity.
Sneezing typically occurs when foreign particles or sufficient external stimulants pass through the nasal hairs to reach the nasal mucosa. This triggers the release of histamines, which irritate the nerve cells in the nose, resulting in signals being sent to the brain to initiate the sneeze
In summary, sneezes serve the role of expelling foreign particles or signalling the presence of external stimulants.
The first thing to prevent is the autosomal reflex, which is the reflex to sneeze when light sources change rapidly. This is present in roughly 18-35% of humans, but is preventable by, say, this gene having not been passed on to the modern era of aliens.
Now, to deal with the mucus immune system
Mucus is our body’s equivalent to fly paper, it lines the nose trapping the dirt and germs stopping them from damaging the lungs. But it does better than just sticking to them – mucus is loaded with protective proteins that kill and disable germs, like bacteria and viruses.
One way to prevent sneezes is thus, if their protective proteins and immune system in general were much stronger relative to the bacteria and diseases, killing off most of the germs and presences in their mucus. Over time, their sneeze reflex will greatly weaken.
Dealing with the allergies, similarly, most allergies do not trouble them enough to activate enough histamines to trigger a sneeze.
As to how a sneeze would kill the alien, a sneeze would be an indicator that so many germs have filled the nasal mucus that it has already spread throughout the body, and not even the strong immune system of the alien can save them any longer.
A sneeze would thus be a signal the alien gives shortly before death to a powerful disease, either helping to warn other aliens off, or prompt swift treatment through medicine.
If left untreated after sneezing, the alien will quickly succumb and die to the disease.
Appearances and Abilities
The appearance of the alien wouldn’t be much too different from us on the surface, but the stronger/more developed immunity system in almost all aliens means that they will fare much better in space exploration on foreign inhabited planets, and they can quickly detect and react to fatal diseases by hearing each other’s sneezes.
Their society would also heavily support universal mask wearing in pandemics to prevent sneezes from spreading deadly diseases.
Another canonical answer could be stronger immune system, but a weak lung structure, which is punctured by the sheer change in air pressure from the sneeze... but isn’t a banshee cry, heralding bacterial death, a more interesting prospect?
Reverse the Cause and Effect Relationship
IE: They don't die because they sneeze, they sneeze because they died.
Instead of being always open like in humans, the default position of the alien nasal cavity is closed off by a sphincter that is actively opened by muscle contraction to allow air in and out of the nose. A good way to justify this is if you aliens are able to breathe through their skin, and only use their noses to take in additional Oxygen during times of physical duress. In this cases keeping the nose sphincter closed is a good way to prevent infections which also explains why normal sneezing is not a typical thing for them.
However, when they die (or perhaps get fatally ill) gasses start to build up in their body and gather in the nasal cavity. Once the pressure reaches a certain point, it forces its way out through the nasal sphincter in an activity similar to a sneeze.