The sequence here is: (Assume that you just resolved Scar or something to put a -1/-1 counter on a random 1/1 creature. It can't be the "one token" above.)
1. Nest of Scarabs triggers and will go onto the stack the next time you get priority.
2. The creature dies when SBAs are checked, and Blowfly Infestation triggers.
3. You put the triggers from Nest and Infestation onto the stack in whichever order you choose. Infestation's ability requires you to choose a target at this point. (Notably: The token you will get from Nest's ability doesn't exist yet and can't be chosen as a target.) If you have another token already, let's say you target that.
4. Let's say you arranged the triggers so that Nest's ability will resolve first. It does and you get a new token.
5. Infestation's ability resolves, putting a -1/-1 counter on the targeted token. Repeat from step 1.

Note that both Blowfly Infestation and Nest of Scarabs have mandatory actions, so to break the loop you will need to do some different sort of sequencing. There are two ways to get out of the loop without some sort of outside effect.
1. If there is a creature with 2 or more toughness (such that it won't die from the -1/-1 counter), target that with Blowfly Infestation's ability.
2. Order the triggers in step 3 above so that Blowfly Infestation's ability resolves before Nest's. If there is exactly one X/1 creature on the battlefield and this one is the target of Blowfly Infestation's ability, then it will die, trigger Infestation again, and this ability will be removed from the stack because there is nothing to target. If there are multiple X/1 creatures on the battlefield, repeat this process keeping Infestation's trigger at the top of the stack at all times until there are no more creatures left to target - after which you'll get a bunch of tokens from however many times Nest triggered while you were doing this.