Everything in real life wants to kill you. Walking down the street in real life, you could be killed by any number of things. These two plugins are really cool and provide immersion.
Because of the Mikotese Pox pandemic they continuously battle, that nation consistently trades Ilara for melon seeds and other cures. This fact here provides a deeper level of roleplaying and history to nations' history.
Ilara struggled with mobs, overheating (czardeath, hahaha) and other setbacks and that is a part of our history. It has made us grow and marks us as survivors.
Pretty self explanatory. You log into the Realms MC and immediately see the Temperature and Disease box and know that this isn't your everyday server. This is unique to this civilization experience. This makes the world that much more difficult and immersive.
Adding the ability to grow immune and tolerance systems will reduce a lot of the grinding that diseases and temperature could bring. For example, after weeks of playing, players who live in consistently hotter environments (like the desert) should be able able to wear iron in that biome and not have a temperature of 9001.
There have been entire corporations established who are interested in creating the vaccines. It'd be cool to make incurable diseases that players have to get the vaccines for.
I think people are already getting bored of the diseases that have been there since day one because there was no real challenge after we learned the basic cures. If we had vaccines like the suggestions above, maybe even make it like brewing where you have to take time to experiment in alchemy to find cures, there would be a revitalization in this plugin.