I'm looking to create a lethal disease for a sci-fi book. The plot surrounds a group of rich people taking teenagers off earth to start a new life away from this illness. The pandemic has been going on for a few years already and there's no cure yet.   

**Some ideas I came up with:**

* A [coronavirus](https://en.wikipedia.org/wiki/Severe_acute_respiratory_syndrome_coronavirus_2#Infection_and_transmission) mutation, more deadly than existing ones. I wasn't sure how it could be made to lie dormant and what would cause it to reactivate afterwards.
* [Rabies](https://www.cdc.gov/rabies/symptoms/index.html). There's a pretty long incubation period, which I liked. Could I make it contagious somehow? Also, I'd need it to be non-curable.
* [Klebsiella pneumonia](https://en.wikipedia.org/wiki/Klebsiella_pneumoniae#Transmission)e infection made contagious so that it infects people's lungs and that resists treatment and constantly gets worse? I wasn't sure how this would work.
* [Kuru](https://en.wikipedia.org/wiki/Kuru_(disease)) disease. I wasn't sure how to transmit this without cannibalism. Also, the incubation period is 10-50 years, which I'd need to shorten.

**Requirements:**  

* Anyone can be infected by it. The disease can stay dormant inside of them for a few years so no one knows if they already caught the disease. However, at least half the population hasn't been infected yet. Tests exist but are very expensive.
* Children aren't endangered by it but are often carriers. They are infected more easily and also infect others more easily. Teenagers can be infected by it but don't develop symptoms until they're in their mid-twenties
* All adults (from roughly their mid-twenties) are in danger
of developing the symptoms of this disease and dying from it (approx. 90% death rate).
* There is no cure and those who do recover are in danger of being infected again in the near future.
* I don't care how it was originally started. I'd like it to be all over the world, now, with no cure in sight, only ways to delay the deaths of those who have it.
* It needs a name.
* Ideally, I'd like to base it off an already existing disease or virus and with create the disease I'm looking for with realistically mutating it.
* The most practical way of transmission seemed to be through the air (through droplets) by coughing, sneezing and (to a lesser degree) talking and through contact with contaminated objects and surfaces, then touching eyes, nose, or mouth before washing hands, I'm completely open to other suggestions on this point.

The main complications in creating this pandemic are:
* The somewhat slow infection rate (I'd like people to still be able to safely assume they don't have it)
* Reactivating the disease. How does it reactivate?
* Only adults are in danger. Children's immune systems are weakest, so they are most likely to be endangered by such a disease.   

Any suggestions on how to alter the sicknesses, or for any other diseases/viruses etc. that could create such a pandemic?