Three conditions for applying the Precautionary Principle

I think COVID-19 is “probably like the flu”, and simultaneously I am in favor of taking drastic action to control its spread. Precaution is warranted here.

This raises a question: When should we be concerned about a future threat?

Bryan Johnson has a fantastic tweet hypothesizing why COVID-19 caught the world by surprise:

Number three is tricky. How do we know something may go exponential? As a first approximation, we can study history. We know many diseases spread exponentially.

The precautionary principle states that we should use excessive caution when dealing with something new. We can’t always be this careful — it would be impractical. However, when all three conditions listed above are met, it seems wise to be cautious.