Sims do not automatically know they have "Crabs" without taking action. You can manage the condition through the following steps:
: While the condition is active, other Sims may notice the scratching behavior, though it does not typically result in a severe social "scandal" unless specific personality traits or settings are active. Diagnosis and Treatment WooHoo Crabs - WonderfulWhims
If you're looking for a format or a statistical breakdown of transmission rates, Sims do not automatically know they have "Crabs"