Before I explain this, here's the basics. For my first Homebrew thing, I wanted to make a system for infection for a zombie apocalypse setting. The twist is that I wanted the infection to be something almost like a drug, akin to the likes of Venom from Marvel. As a result (and as the infection is based on a homebrew dark god), I will mention Parasites as they are the main ways to spread it. If I get the time, I may write a statblock for both Parasites and Hosts. Anyways...

When attacked by a Parasite, you must make Will Saves as Infected. This functions like Death Saves, but you are still conscious and are able to do half your movement and either your action or bonus action, as the rest of your turn is spent on the saves. If you succeed, you can wipe off the Parasite with an action (Although your allies can try to kill the parasite but harm you in the process). Upon Success, you are now considered Infested.

Infestation

Adrenaline Overload

The Parasite puts your systems into overdrive. As a result, you are always under a level of exhaustion. However, you have advantage on saves, attacks, and any skill check involving Strength or Dexterity.

Daily Deal

The Parasite tries to lower your mental guard when you try to sleep. Every dawn, make a Will save (just like infection) and if you fail, you take a 1d4 penalty to wisdom, which stacks, and if you succeed, you get an Infection Save. Get 3 Infection saves within a month and you will be able to wipe the Parasite off, recovering the lost wisdom in the process. If you die due to Wisdom being 0, you will be brought back as a Host within 1d6 hours. (If I don't do statblocks, they have a weakness to Radiant damage, has a weakness to Necrotic, and has the same stats, skills and equipment as the original user).

Help from Beyond

As the Parasite needs you to survive at the moment, it helps you fight death. As a result, upon being brought to 0 hp, you are able to be brought back to life using all your hit dice to heal to that point and taking 1d4 penalty to Wisdom, skipping the Daily Deal for that day.

Radiant Aversion

When you take Radiant Damage, you can make a Wisdom save (DC is 8+prof.) to see if the damage effects you. This save is only done once a day, resetting when you use a Daily Deal. Upon success, you get an Infection Save (see Daily Deal) and upon failing, you take double Radiant damage for that hit. However, you view sources of Radiant damage (like Paladins or Clerics) as threats, as they put your connection to the parasite at risk as you view the parasite as helpful.