Grimm Plague

Transmission is a little inconsistent, the general commonality is that victims felt negative emotions before infection, normally when attacked by Grimm like the Hag.   The Hag can hex victims and infect them with the plague and can often be seen shadowing the Apathy and thus worsen it's effects by infecting victims with the Plague. Sometimes the consumption of black blood or black blood entering the blood stream can cause people to become infected.

Somehow when the Victim is infected their DNA is attacked by Grimm parasites that weaken the body and eventually rewrite DNA and turn people into Grimm. Whitecliffe's Department of Health believes that some black blood contains micro Grimm which are the parasites and find viable victims by detecting their negative emotion. As for hexes, the Hex creates miniature Grimm pools on the victim in order for the parasite to infect the victim.   Once infected the theoretical Parasite find their way to DNA strands and begins the dreams, Neurologist Stefan Writer  noted in his study in 78 that victims seem to have their brains infected by the Parasite causing abnormal brain activity that is similar to dreams. It is also said that the parasites are likely invisble or hard to see, else they would have found the parasites by now and no other explanation of infection is viable.

The Plague has multiple stages, here are the big 3:  

1. Infection

The Victim is infected somehow and gains a mark, this can vary between thorns, Veins, Tendrils and webbing and will slowly spread, exasperating negative emotions of the victim making them more visible to Grimm which Grimm will sue to find their friends and kill them.  

2.Nightmare

Most Infections have a nightmare stage, this is because Aura needs to be consumed in order to fuel the death of the Victim and subsequent transformation. When falling into a nightmare the victim is mentally tortured and slowly made to give in or embrace corruption. Dreamwalkers can stop the plague at this stage.  

3.Transformation

If left untreated or uncured, the Victim will run out of Aura and die, their body will then violently mutate and the parasite now fully formed will grow and take control of the body, fully possessing the Victim, leaving the only recourse to be Blood shed in order to prevent further infections or death.

This is a difficult question, Scientifically there is no known cure for the plague and most victims under medical care end up nearly transforming and have to be put down to maintain the Victim's Dignity. There is a mystical path but these people are few and far between and often thought to be scammers, and some are. These people are Dream walkers.   People with Semblances that allow them to manipulate dreams in unique ways that enable them to defeat the parasites and save the victim in stage 2, there is precedent of this but with so few available and so few of that minority legitimate, this is extremely unlikely to happen and is considered lucky. Some crazy people have resorted to drinking the victims blood so they too will be infected and enter their friends dream in hopes of rescuing them before they too die.   Another solution is to find the Origin Grimm who infected you and hope that killing them will cure the victims as the parasites in their bodies dying as a result of termination of the patient zero Grimm.   Sadly outside of Suicidal Strategies and Mystical Solutions, if you become sick with this, you will die, end of story and there is no medicine that can be used to allievate your suffering, causing people to put themselves out of their misery before the infection gets too serious.   Maybe one day, someone will innovate and find a workable cure to stop all this death.

There are ways you can stop this however, semblances, charms and simple relaxing music can be used to lower the stress and thus make you less vulnerable to infection. Emotional control while combating Grimm is paramount. Wearing Gas masks, protective armour and equipment can also lessen the likely hood of infection.   This is why so many Hunters now wear face masks, helmets, and protective stuff to prevent ingestion of Black Blood or getting some in their eyes or worse.  

It's really hard to track how many people are sick as so many types of Grimm could potentially spread it, this means that no one has any real clue just how many people are dying from the Plague and if there is a Outbreak, it will likely spread quick as Panic will make you vulnerable to infection and panic is natural in viral outbreaks. Most people who get sick are quarantined and put somewhere where they can safely die without killing more people.   If truly out of control, it could potentially wipe out entire civilization, causing swarms of Grimm to spawn within cities and destroy any level of safety they may have achieved.