According to spoilers, Ava sends Steve on a wild goose chase, for exactly the reason soapie stated. She wants him out of the way, so she can do harm to Kayla, possibly Joey as well. She is not ill, she is faking that completely. She planted the pics, dropping them purposely for Steve to find.
Years ago, Steve was thought dead by his family, but had been held prisoner, tortured, and had amnesia, was living in Cleveland, when he & Ava hooked up for a while. Brother Jack happened upon him, brought him back to Salem, lots of story til he regained his memory. Then, now several years later, he, Kayla, Hope, Bo, ended up at a compound where Ava's gangster father was keeping her, drugging her to keep her compliant, I guess. In trying to get everyone freed, Steve pretended to fall for Ava again, Kayla found them in bed. Long story.
We are only guessing now, Ava is pretending she had Steve's baby, maybe gave it up for adoption, etc. Who knows. Her object is to get Kayla out of her way. Or else to harm Kayla to revenge herself on Steve. ???