The "why now?" is probably due to a favorable set of circumstances that initiated the jump from animals to humans. The SARS coronavirus probably exists in some animal species, very possibly the civet cat. There is no evidence that the virus causes harm in the civet cats. In fact it might be like related coronaviruses in humans - people get colds and the virus mutates, but remains under control my the vast majority of people with relatively intact immune systems.
It is likely that previously the civet CoV wasn't able to replicate in humans, so it wasn't a problem to the small number of people exposed via raising or slaughtering. However, it seems likely that there was a mutation that allowed the virus to replicate in humans, and humans were infected last fall.
Once in humans additional mutations could take place in this new virus (although it is a coronavirus, it is quite distinct from the ones that cause colds). One of its novel genes lost 29 nucleotides, which may have created the current epidemic. All but one (GZ01) published SARS coronavirus has this deletion. Moreover, the gene has been hit a second and third time already, so it seems to be important for creating a lethal infection (it is something like a mini version of the E2 gene which determines host range).
The sudden decline in SARS cases suggests there is a seasonal component. This may be related to propagation of the virus itself or co-factors. Co-factors are high the the list of suspect agents because even when SARS was spreading quickly, most of the spread was by a small number of individuals (super-spreaders).
I think the virus is currently spreading, but only causing mild disease because the seasonal co-factor(s) are missing. Next fall/winter will bring together the mutated virus and the seasonal factors and then it will be quite evident as whether to the quarantine efforts put SARS back in the box, or if the epidemic came under control because of external factors. |
