Dead Frog Disease (5e Disease)
From D&D Wiki
This disease is a deadly disease only found in wet tombs or deep in sewers. A simple frog carries this disease. The frog is either a very dark green or brown and has huge deep yellow warts all over. There pupils turn white and they smell terrible. If a player is touched by this frog they must make a Constitution DC 15 or be infected by the disease. It slows the player down. Their speed drops to 1d4+2 feet (My group did it 1d4x2 feet). Their movements and reactions time become slower and they have disadvantage on all DEX rolls. And in battle they lose 1d4 Hit Points at the beginning of each turn. It is curable by either an ancient ritual that takes a day to complete. Or curable by how you see fit.