Bleeding fever

Symptoms:

Bleeding fever is an extremely deadly and horrible illness. It causes a high fever, requiring someone to take care of the ill person. But the much worse part is why it was named bleeding fever. It causes veins to basically pop open, causing extreme internal, or sometimes external, bleeding, which resulted in death in all known cases, even just a day in.

Location:

The disease spreads among all Children of Myths and can be found on any populated landmass with contact with the other populations.

Living conditions:

The virus thrives in wet, temperate climate and can survive in cold enviroment. It can't handle hot and dry enviroments. It is able to form a hard shell around itself and survive climates that would be deadly for it otherwise. It seems to stay dormant, mainly becasue of the tropical climate, rarely waking up.
This is probably why catching the disease is so rare. Perhaps a large portion of the population has it, they just don't know about it. If the climate changed for a few days to accomodate the virus, perhaps it could kill entire settlements.

Spread:

The virus spreads through liquids like blood or saliva, or drinking water.
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Cure:

There is no known cure, but theoretically, if the bleeding could be stopped, it would kill far less people. Changing the climate for the person seems to not be fast enough for the virus to go dormant, plus the body won't be able to stop the bleeding fast enough.
Though with enough care, it could be possible if caught early enough with a good immune system and perhaps a drug that reduces bleeding.