Regardless where you put your colony to, if you keep up a transportation system to that colony and back, you end up enabling spread of the disease. Coronavirus only spread so quickly because of airplane travel. Had we cancelled all airplane, train and ship traffic to and from China early enough, the virus wouldn't have spread as quickly. If in the future we have an intergalactic society with FTL travel, and one colony discovers an ancient virus that kills everyone, the virus will spread with speeds faster than light, because that's our underlying travel method.

To meet threats like coronavirus, you don't need a different planet. Any remote island would do, like easter island, as long as you shut down traffic soon enough.

New transportation keeps reducing the time delays for trips, but if you’re talking other planets, how fast do you think we can get? I expect a lull.

Of course if panspermia turns out to be true, we could discover some new branch of life that medicine or mammalian immune systems struggle to identify, but which likes to chew on bones or collagen, turning us all into jello.

I generally agree, but a self-sustaining moon colony is far more practical, assuming water can be found there:

1. plenty of solar power

2. close to earth

3. easy to launch out of its gravity well

4. lunar base technology can be iterated far more rapidly

Making a reasonable-sized, sustainable colony on Mars would arguably be significantly more work than considering and planning for "large-scale, exponential, and extinction-level events".

There is no point in having a "backup copy" of society when we're dead. Colonizing Mars would only help the tiny minority that lives there. All efforts are better spent here on Earth.

Elon Musk's endeavors are more about vanity, I think. I found all the twitter antics to be quite odd for someone in his position.