Grey Water Reuse: Safe or Not?

I recently attended a reuse symposium, where reuse of various types of water were discussed.

One speaker spoke of how wrong and absolutely risky it is to reuse grey water if not treated before reuse. She was emphasizing the extreme health risk it poses for direct reuse.

A following speaker spoke of how they directly reuse grey water to water gardens and flush toilets in a community project. This speaker said that there are no serious health risks to this application.

We in South Africa are experiencing a serious drought. South Africans are reusing shower/bath water to water gardens or flush toilets in our area.

So my question is - is it safe to reuse grey water for gardening and flushing toilets without treating? What health issues could using grey water in gardens pose? Is there anyone who can shed light on where we can we use grey water without pretreatment?