It's pretty hardy and goes dormant. You only need a few alive to grow a healthy starter back when you need it again, so every now and then I make an extra amount, mix in a little more flour than normal, spread it in blobs on an aluminium foil-covered baking tray and stick the whole thing in the freezer. After a few hours I come back, pop them off the foil and into a freezer bag.

When reviving, I try to use two blobs from two separate bags just in case one has died. I put them in tepid water for a bit, then add flour and mix and I'm back in business.

My starter was given to me by Dan Lepard[1] (the end-boss of breadmaking) during a sourdough masterclass that I was given as a present. I thoroughly recommend this as a present if you have a friend or loved one who is interested in making bread btw. It's fantastic and you get payback in tasty loaves for many years.

[1]https://en.wikipedia.org/wiki/Dan_Lepard

Freezing doesn't kill bacteria, it's actually how biologists preserve their samples. Most yeast you buy in a store (e.g. Fleischmann's) are better if you freeze it after opening for best quality.

At one point in the past, I dried mine out and put it in an envelope in a drawer. I was able to reconstitute it from this at a later point. The starter I have came from here: http://carlsfriends.net, for anyone interested in sourcing their own.

You can buy packs of frozen yeast from most bakeries.