Nero Claudius Drusus Germanicus, born Decimus Claudius Drusus and variously called Drusus, Drusus I or Drusus the Elder (38 - 9 BC) was the younger son of Livia, wife of Augustus Caesar, and her first husband, Tiberius Claudius Nero. However he was born shortly after Livia had divorced him and married Augustus. So rumors suspected Augustus to be the real father, providing his motivation for a sudden marriage to an already pregnant married woman. According to Suetonius, he was born with the praenomen Decimus, but it was later changed to Nero.

Drusus later married Antonia Minor, the daughter of Mark Antony and Augustus's sister, Octavia. Their children were Germanicus, Livilla, and the future Emperor Claudius.

Drusus, like his brother Tiberius, was a skilled general, campaigning in Germany and along the Danube. He died on campaign against the Marcomanni in consequence of a fall from a horse. He was extremely popular with the legionaries, who erected a monument in Moguntiacum (modern Mainz) on his behalf. Augustus later wrote a biography of him which does not survive.

Genetic Evidence that Augustus was the biological father of Drusus

Augustus was infamous for being chronically "sickly" (although he lived to be 77, and records of the symptoms he suffered from match those of Wilson's disease, an autosomal recessive genetic disease).

Augustus only had one known biological child; Julia Caesaris. She gave birth to the five grandchildren (3 boys, 2 girls) of Augustus, whom he adopted as his heirs. The elder two boys died before they came of age, and the third was considered unsuitable (perhaps he was mentally ill). One of these granddaughters of Augustus was Agrippina the Elder.

Tiberius was already 4 when Augustus married Livia, while she supposedly was already pregnant with Drusus. Drusus' son was Germanicus, and Germanicus married Agrippina the Elder. Their son was Caligula.

Caligula, often called "the Mad Emperor", was affected by more severe symptoms that match with Wilson's disease; apart from the intestinal problems and immune system weakness of Augustus, he was mentally unstable and suffered from psychosis.

Because Wilson's disease is recessive autosomal, each of his parents would have to have been a latent carrier for the disease. It is entirely possible that Agrippina inherited the allele for Wilson's disease from her grandfather Augustus, but the only way Caligula could have Wilson's disease is if his father Germanicus was also a carrier. As it is a rare disease, it is unlikely that Germanicus inherited it from his normal-type mother. Thus Germanicus had to have been given the allele by Drusus, and the only way that Drusus could have the allele for Wilson's disease is if he got it from his biological parent: Augustus. This linkage shows that Augustus was Drusus' biological father.

Nero also probably suffered from Wilson's disease, and he was the son of Caligula's sister Agrippina the Younger and Gnaeus Domitius Ahenobarbus. Claudius, brother of Germanicus, probably also suffered symptoms of Wilson's disease, lending further proof to the genetic mapping. (Compare this pedigree to that of, say, the presence of the hemophilia gene in the monarchs of Europe descended from Queen Victoria).