I haven't dulged deep into the offspring of purebloods and lycans but I'll give a back story to the disease and if you can make a logical reason that sounds good to me to how someone could be born without it that's fine.

[i]Back Story[/i]

Before the collapse the grounds Urbi Lupi is built on was a complex of labs both bio and tech. One of the teams at those labs was focused on finding a way to genetically improve humans by mixing their RNA with that of animals. These studies were never finished and the vials of hundreds of animals were stored away. Over the thousands of years that passed nearly all the vials were destroyed or contaminated by nature. One vial however survived, one with timber wolf blood. One that as written in a journal by one of the scientist, was considered too dangerous. One that caused extreme violence in test subjects.

 One of the early settlers of Urbi Lupi before it was a city stumbled across this lab and by mistake, stepped on that very vial which had fell out onto the ground. The glass shattered and cut the man's foot. Mixing his blood with that of the genetically modified timber wolf blood. The RNA was combined with a very aggressive virus and the lycanthropy disease was born.

For years lycans were seen as no longer being human, and a threat. There was also a misunderstanding that the moon had something to do with the transformation process but as it turns out the moon had nothing to do with it. Transformation is actually triggered by strong emotions such as sadness, anger, and fear. So it was due to the fear of the rumor that people would change on a full moon.

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Theoretically the virus should be passed down by blood, saliva, or genetics. Also people who easily get mad or suffer from depression or anxiety would easily change without meaning to. People with more control over their emotions could hold back this change.

Do with that info as you will