Are inactive infectious diseases or toxins detectable 5, 10, 20 years later if treated or untreated?whats recorded or left behind in dna/blood/or other

Disease markers: Infectious disease markers are used to identify patients with many diseases, such as hepatitis, tb, and hiv, and some also may indicate active infection. Some markers are indicative of previous (but not always "inactive") diseases such as syphilis, which is often overlooked until late stages unless the proper tests are undertaken for specific reasons (symptoms).

Generally yes. : When one gets an infection, the body responds with the immune system getting revved up to fight it. This can involve antibodies to the foreign agent (called antigen). This become useful in certain entities; an example would be the epstein barr virus, which causes infectious mononucleosis; it is also the virus which is thought to be involved in chronic fatigue syndrome.