I understand that's why the government would want to know. My point (thus the word "need" in my question) is that at least in the United States there are very few groups of people (largely healthcare/childcare workers) who are required to disclose to the state. You're frequently required to disclose to partners, but that's obviously a completely different thing. Even then, disclosing where you got it from is not a legal requirement, merely the fact of the infection.