Rare infectious diseases are caused by external factors like genetically or through the environment. Rare disease affects a small percentage of the world population. In some countries, rare diseases are called orphan diseases. Whose rarity means there's a lack of a market large enough to gain support and resources for discovering treatments for it, except by the government granting economically advantageous conditions to creating and selling such treatments. Some of the rare diseases are rat-bite fever, seal pox, scarlet fever, meningococcal diseases, bubonic plague.