Interaction with surroundings can bring about infection
Customisable diseases and remedies
Disease will ignore players in creative
Temperature and sickness will go up as time passes
Messages at certain levels to tell players of danger
Effects applied to player as they get more unwell
Some Diseases are Infectious and can spread to other players
Cured by bandages, curative potions or time & player action
Allow players to apply cures to other players
Heart particle effect on cure
Vaccinate players against disease
Ability to craft curative potions and gels
Ability to craft bandages to cover wounds
Ability to craft leg splints to repair broken leg
Craft syringes to extract blood and inject vaccines

Temperature Effects:

Heat - fatigue, confusion
Cold - increased chance of pneumonia, freezing slowness, blindness

Stay cool by wearing less in warm climates, sheltering indoors in desert sun, not holding hot items such as lava buckets, and going out at night.
Stay warm by wearing more clothes in the cold, lighting campfires, carrying hot items, staying out of water, and stay indoors during storms and at night.

Armor or food / campfires etc can interact with temperature effects.