The moisture problems behind rental mold
Mold grows wherever there is extra moisture and little airflow. In rentals, the usual causes are plumbing leaks, roof or window leaks, flooding, and poor ventilation — most of which are the landlord's responsibility to fix.
Mold needs three things to grow: moisture, a surface to grow on, and time. That is why it shows up around leaky pipes, under sinks, behind walls, in damp basements, and in bathrooms without working fans or windows. Condensation collecting on cold walls and windows can feed it too, especially in units that trap humidity.
Because leaks and building ventilation are things the landlord controls and must maintain, the root cause of rental mold is often something the landlord was supposed to keep in good repair. Understanding the source matters, because fixing the mold without fixing the cause just delays its return. If you can trace mold in your home back to a repair your landlord neglected, it may be worth learning about your rights.
This is general information about California tenant rights, not legal advice. Every situation is different.