Re: Where did smallpox come from?

Smallpox is a disease caused by the variola virus, a member of the pox virus family, a group of large double-stranded DNA viruses.

Taxonomy:

Subfamily:																Genus:													Members:
Chordopoxvirinae
																										Avipoxvirus
                                         			 fowlpox virus
                         	Capripoxvirus
                                          			sheeppox virus
                         	Leporipoxvirus
                                          			myxoma virus
                         	Molluscipoxvirus
                                          			Molluscum contagiosum
                         	Orthopoxvirus
                                          			vaccinia virus
                         	Parapoxvirus
                                          			orf virus
                         	Suipoxvirus
                                          			swinepox virus
                         	Yatapoxvirus
                                          			Yaba monkey tumor virus

At least 9 different poxviruses cause disease in humans, but Variola virus (VarV) and Vaccinia are the best known. Monkeypox also produces a disease clinically identical to VarV (but not fatal) - several hundred cases seen in Africa in the last decade. VarV strains are divided into Variola major (25-30% fatalities) and Variola minor (same symptoms but less than 1% death rate). Smallpox was probably the most important disease in medical history in terms of fatalities and was rife in many countries. The first cases of smallpox to be reported came from China and the Middle East more than 2000 years ago. The first type of "vaccination" was practiced by the Chinese and was called Variolation.

"Variolation" = the administration of material from known smallpox cases (hopefully Variola minor!!!) to protect recipients - practiced for at least 1000 years but it was risky

Edward Jenner was nearly killed by variolation in 1756! Edward Jenner, 14th May 1796, used cowpox to "vaccinate" 8 year old James Phipps, who he later challenged with VarV and showed that he was protected. For more than 100 years, the "vaccine strains" were propagated from arm-to-arm and from cows, but for at least the last 50 years, Vaccinia has been a distinct virus from Cowpox (In molecular terms, vaccinia is most similar to Buffalopox). Infection with VarV and Vaccinia occurs naturally by the respiratory route (high transmission rate!!!) and is systemic, producing a variety of symptoms, but most notably with VarV characteristic pustules and scarring of the skin. Vaccination was almost universally adopted worldwide around 1800, but it took a major commitment from the WHO in 1965 to achieve eradication. Eradication of smallpox was possible for 3 reasons:

1) There is no other reservoir for VarV but man (including primates) 
(c.f. Arboviruses) 
2) VarV causes only acute infections, from which the infected person either: 
       a) dies 
       b) recovers with life-long immunity (c.f. Herpesviruses) 
3) Vaccinia virus is an effective immunogen. 

The million dollar question is the one you asked. Where did smallpox come from? The truth is that nobody knows. One hypothesis is that VarV evolved from another related virus which has may or may not ceased to exist and may or may not been from the same species (e.g. Monkeys). Another hypothesis is that VarV is derived from another organism, such as bacteria, a long time ago and has mutations that means that it has to enter cells to replicate. Another possibility is that it evolved from/like eukaryotic cell organelles such as mitochondria.

The possibility of this happening again is possible but VarV is highly unlikely (almost impossible) to be recreated as a new poxvirus would have to evolve in exactly the same way with exactly the same mutations and meeting the same hosts. An altogether different poxvirus could be generated from two different strains of poxviruses from different hosts by recombination of their genomes which may be pathogenic in humans.

For a short tutorial on Smallpox you can go to A dose of Pox