I believe that not only was Ruku inspired by a real life disease, but it is more contagious than revealed in canon.
Let’s start with the name and description of Ruku. Rukuta was dubbed “The Silent One.” If a disease is silent, typically that means it kills fast but goes unnoticed. Typically, illnesses like these have long incubation periods (the period of time between getting infected and showing clinical symptoms). A great example of this is rabies. You can harbor the rabies virus for years after being bitten by an infected animal, but once you start to show symptoms, there is no treatment and you die within 3-10 days.
Considering Blackgaard faked his death and then seemed totally fine afterwards suggests that Ruku does have a long incubation period. Blackgaard only started to get sick when he was campaigning for mayor, which was probably quite awhile after he got back from Switzerland. He only started to really go downhill in the last few episodes.
A lot of diseases have a long incubation period, so this alone means nothing. However, we also know about its transmission. Blackgaard’s whole goal was to get the TA-418 mineral. Why? Because TA-418 was what made Ruku airborne. This would mean that Ruku is not naturally airborne, and is a strict anaerobe (okay technically the terms “aerobic” and “anaerobic” only apply to bacteria and not viruses but you get the idea).
Viruses that die in the open air are not that uncommon. Of course some viruses are only spread via ingestion, like stomach viruses. But Ruku cannot survive outside of its container or a host. Viruses like these are called “Direct Contact,” or “Bloodborne.” Hepatitis C is a good example. You cannot get hepatitis C by existing in the same space as a person who has it.
Ruku never got a cure. It was a death sentence as no treatment or antidote existed (credit to Bovril for trying though).
Also the Blackgaard Chronicles takes place in like the 80’s & 90’s.
So now let me ask you: What bloodborne disease has a long incubation period, was very prevalent during the 80’s and 90’s, and had no cure, almost always leading to death?
DING DING DING! HIV/AIDS.
Was Ruku meant to be a carbon copy/reflection of HIV? Probably not. It’s different in its own way. But the similarities are undeniable. I’m sure more diseases went into the inspiration of Ruku and it was not based off of HIV/AIDS alone. Based on the symptoms described in the audio (Blackgaard passing out, being very weak, noted as being exceptionally pale, and not eating), it would seem that Ruku would be a gastrointestinal illness, probably causing extreme lack of appetite, malnutrition, and dehydration. However, it is clearly not incapacitating like Norovirus or Cholera, where you’re stuck in the bathroom for 3 days (norovirus) or until you just get so dehydrated you die (cholera). It’s slow, controlled. Blackgaard was still able to live relatively normally day to day, he was just extremely weak.
I really really want to see if the next books touch on this at all. With all the behind the scenes stuff they’re doing it’d be absolutely ridiculous for them to not touch on Ruku. What is the disease progression like? Did Blackgaard have any other symptoms that we didn’t know about in the audio? Did he die when the disease was still in its early stage or was it already pretty advanced by The Final Conflict? Did Nicky know?
Also hypothetically it could still be contagious. Tasha said that the only way to get infected was to have the virus injected directly, but we know this is because it’s bloodborne, meaning it could probably be spread the same ways as any other bloodborne diseases.
Imagine Blackgaard pulling a pro gamer move by donating blood right before he dies knowing that the recipient is gonna be in for a fun time.
Imagine being that poor guy who just got a life saving blood transfusion but you just get Ruku and die because nobody knew it was contaminated 💔