USAID is an arm of the US intelligence community. Every US embassy has a CIA Chief of Station. That most USAID employees and cobtractors are sincere and not officially or knowingly spies does not mean that they can really be trusted. They're agents of a foreign power and they're in your country to further its hegemony.

Remember how they found bin Laden? If they'll use a vaccination programme as a front to spy they're not going to take the "independence" or "mission" of USAID seriously.