Tuberous begonias are rather prone to do that. I'd be surprised if your other plants "caught" something from it, usually different plants vary in susceptibility. Just to clarify, these are all under a roof so being hand watered? Is it tap water or do you collect rainwater? Has anything changed in their care, for example could someone else be watering them and giving too much? Has the water source changed, or the fertilizer? Has anything else changed - repotted for example?
to NGA.
