I personally would cleanse every 3 months if i need to. meaning if i was sick ect...
if your not having issues then twice a year is good. if you r having a serious issues with your health it will be more frequent to cleanse obviously.
parasites are very contagious. My ex girl friend was infected from alot of parasite we had when we were together and still is infected. it really depends on the strength of the other persons immunity. if you caught a parasite from your BF
typically you would never know because your body would keep in check. But if you are taking alot of birth control or any type of hormone treatment it could set off any parasite infection. Sushi eaters 100% have parasites no doubt but their bodies keep them in check.
p.s-cat/dog feces can transmit parasites and also saliva and eating after animals as well. it not advised to do that.