She is a Gravure Idol which means Dresses and swimsuits (NO NUDES) she does many photo shoots and they are released as Photobooks, DVDs, and other merchandise (calenders, cards and such).
Now someone compared this to Takeru but remember he has some series acting experience and even he was felling the stresses of the show, where this is Yuriko's first time out. Now somewhere around 50 episodes + a movie + special + all the promo work and ADR , and in addition all the work she has to do for her agency would make anyone tired and susceptible to getting sick. I'm glad to hear that shes felling better and will resume working, which means it was nothing serious.
A thought popped into my head, we also don't know what kind of stresses the company Execs are putting on them to get better ratings, that in it self could lead to stress.