No disease—but far from harmless
Bed bugs are not known to spread disease to people. Their main harms are itchy bites, allergic reactions, skin infections from scratching, and the sleep loss and stress of dealing with an infestation.
Unlike some pests, bed bugs are not considered disease carriers. That does not make them harmless, though. The bites, the risk of infection from scratching, and the anxiety and exhaustion of living with them are genuine problems. For people who have strong allergic reactions, bites can be more serious and may need medical attention.
If you are worried about a reaction or an infected bite, see a doctor — this is general information, not medical advice. The lack of disease transmission does not lower a landlord's duty to treat a serious infestation, since bed bugs still make a home hard to live in. If yours will not act on the problem, a free review can help you understand your rights.
This is general information about California tenant rights, not legal advice. Every situation is different.