Short answers, pulled from the story.
The Historia Augusta is a late Roman collection of biographies that Isaac Casaubon established as the modern name in 1603. Before this publication, the text appeared under different titles within manuscript traditions like the Codex Palatinus written during the ninth century.
Hermann Dessau proposed in 1889 that six authors were fictitious personae and the work was composed by a single author in the late fourth century probably during the reign of Theodosius I. Most scholars now accept the theory of a single unknown identity writing after 395 though Alan Cameron suggests a composition date between 361 and 380s instead.
The collection contains around 150 alleged documents including 68 letters 60 speeches and proposals plus 20 senatorial decrees and acclamations. Almost all found in the Historia Augusta are rejected as fabrications partly on stylistic grounds and because they refer to military titles or administrative organization unrecorded until long after the purported date.
Table data shows estimated amounts of reliable historical details with primary vitae averaging approximately 17% total reliability across the collection. Inscriptions uncovered confirm existence of figures like Veturius Macrinus praetorian prefect 193 but no other extant ancient writer apart from the Historia Augusta confirms these points.
Biography of Geta states he was born Mediolanum on the 27th of May year unspecified suffect consulships Severus and Vitellius actually born Rome the 7th of March 189. No such pair existed and the text mixes emperor Didius Julianus with legal scholar Salvius Julianus in its accounts.