Physics can break the body. Bioweapons can obliterate it. Painfully. Over a long time. With no chance of reprieve. 

The worst forms can do it so that it remains undetected until it is too late to stop. And if she develops the hoped-for version, it won't be traceable back to her, as the weapon will self-destruct after mission is complete. Just imagine... A small sample is ingested as part of a meal. It is unnoticeable. Over the next time period (week, month, whatever) it attacks something vital, like the kidneys, liver or such, striking slowly, breaking it. Making it fail slowly. When that is done, the virus self-destructs and is naturally destroyed by the body or purged from it the way any other waste product is. Then, a week or a month later, the person slowly weakens as the affected organ(s) slowly ceases to do its job. As a healthy individual, the body can last a while before the buildup of bad things affects daily function. Perhaps long enough for the organ(s) to fail completely. By that time, unless they have a replacement on hand then and there, it is simply too late. 

As this happens weeks or even months after the original insult, most, if not all, have forgotten it. Organ failure can occur in even the healthiest individuals. There are too many things still unknown about the human genome.

Mind you, it takes more than a simple insult to get Ylva to do such to anyone... We think...