They fell ill after leaving the restaurant and never reached their home
So?
How about the following scenario? Yulia returns from Russia with contaminated items in her luggage. Sergei and Yulia become ill after opening same during an outing that Sunday. Perhaps it was a present for Sergei. DC.Bailey later searches their house and comes into contact with more contamination and becomes ill.
Six question marks doesn't make it any more mysterious.
How does our incomplete knowledge of the details become fuel for insane conspiracy theories?