First Aid is a game changing mod focused around a new health system.

HEALTH SYSTEM:

The old health bar got replaced by 8 different health regions, so your Head, Body, Legs and Feet have their own health bar.

This mod disables vanilla health regeneration by default. This effect stays even if you remove the mod. It can be changed in the config or by typing /gamerule naturalRegeneration true.
Healing can be done by using bandages or plaster. When you right click them in your inventory, you get a GUI where you can choose where you want to apply it.
You will die as soon as your head or body health drop to zero.

DEBUFFS:

Certain debuffs may apply when your limbs get damaged, so when e.g. your feet get damaged, you will get a constant slowness debuff, or when your head gets hit servere enough, blindness and/or nausea will apply for a short time.
All debuffs can be supressed by morphine. When you take morphine, no health debuffs will apply while it is active, but as soon as the effect runs out, your constant debuffs will reappear.

ARMOR:

Armor calculation has also been changed: If your legs get hit, only the armor of your legs will be taken into account.
But because the armor value of a typical helmet is much lower than a chestplate, these values get multiplied with a factor
So armor for your head and body are very important if you want to survive.

Since version 1.3.0, this mod uses a sound from this website. It is licensed under the Attribution 3.0 license.

NOTE

This mod disables vanilla health regeneration by default. This effect stays even if you remove the mod. It can be changed in the config or by typing /gamerule naturalRegeneration true.