Industry standard for testing is every 28 days. Tests expire after 30 days. And yes,people can catch something and spread it on within that time. Most of the time they dont even know they are doing so since it often takes more then 28 days for symptoms to present themselves.

Unfortunately...thats the chance you take in this industry. Condoms will help a bit,but some things can be passed even WITH condoms. Plus...condom scenes dont sell nearly as well.