I Am Married. If You Have A Regular Smear Test

I am married. If you have a regular smear test and in year one your test is clear and then Year two you end up with ureaplasma infection which details state is sexually transmitted.

If both partners are only sexually active with each other how does is this bacteria created?

Or if in yr one my smear test is clear and Year two not - does this mean my partner may have caused me to have these symptoms by stepping outside our marriage?