I've had strep 3 times in the last 4 months but each time with slightly milder symptoms. could i be developing some kind of immunity? i ask because i suspect that my new partner is a strep carrier.

You may be too: Depending on the time of year 5-20% of the population may be asymptomatic carriers. Although a positive quick test during a throat is considered a strep diagnosis, many may just be carriers with a virus. The blood testing needed to confirm the strep status is impractical & invasive. Have your partner tested. There are things that can reduce the carrier state.