This includes spoilers for Infinity. View at your own discretion.
Why do I die instantly?
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Is itUnlimitedDamage? That means you didn't disable the era shield - era still exists in infinity, so enemies still have all the benefits of era phase - including unlimited damage.
Is it a tiny number, like0.1or0.01? These tend to be caused by enemies mimicking your modules. Either don't use, or 'secure' them.
Is it a really big number that shouldn't be there (such as 1e270 in forest easy)? Make sure you have Infinity Foundation in the BP.
Otherwise you'll take a big hit. Look at your BP, try improving it, maybe go to a lower region/difficulty? Keep in mind an enemy can mimic#REACTIVEmodules like Reflect or Barrier, which can be fatal.
Why don't I kill anything anymore?
Infinity gives enemies an 'Infinity Shield', that reduces the damage they take by 1e50 (big number). This is likely more than you even do when starting in infinity, so expect to start out doing ~0 damage. This is what the module Infinity Attack is for.
Also, stick to Easy until you get better tools, as Inf Attack does nothing against shields.
Enemies can cast spells now?
They mimic modules. Some are helpful (to you), some are harmful (to you), some do absolutely nothing. Think carefully about what you use in your BP and how it might be used by the enemy.
Enemies mimic 1 module for each digit of your current infinity - 2 at INF 10, 3 at INF 100, 4 at INF 1000, etc.
What are fragments? What is their use?
Infinity enemies drop 1 fragment for each module it mimics, the fragment type is the mimicked module type. Fragments can be used to purchase infinity modules, as well as boost the Infinity Forge and Anvil. Contracts and various other factors affect Fragment gains, so be on the lookout!