There is a ping pong effect if an infection and/or sexually transmitted disease goes back and forth between lovers.
This happens a lot with vaginal fungus infections. The girl has symptoms and goes to the doctor to be treated. Her lover doesn’t go because he hasn’t got any complaints. If she has intercourse after treatment has been successful, he may give it to her again.
The same thing may happen with sexually transmitted diseases. That’s because you can be infected without having symptoms. You don’t seek help and may without knowing it transmit the disease to your lover again and again.
Curing one doesn’t cure the other person unless the medication is used simultaneously. If you are in a steady relationship, don’t use condoms and you have complaints you should both be tested and treated at the same time to avoid the ping pong effect.