The innate immune system can be boosted in response to subsequent triggers by pre-exposure to microbes or microbial products, which is known as "trained immunity". Trained immunity is an essential protective mechanism that enhances protection against secondary infections, inducing by a variety of inducers, and the main mechanisms are epigenetic and metabolic reprogramming. Innate immune training can involve cancer and infection diseases due to its immunity enhancement after pre-stimulation, but causes maladaptive effects and leads to excessive inflammation and the development of various chronic inflammatory diseases, including cardiovascular disease, autoinflammatory syndromes, and neuroinflammation.