3. Potential Outbreak Scenarios for SARS-2
Having identified some relevant natural and social factors common for affecting both SARS epidemics, it is also necessary to discuss if variations in these factors contributed to the unique outbreak of SARS-2 in Wuhan. Because many factors confounding the SARS-2 epidemic are still unknown, we herein discuss SARS-2 outbreak in Wuhan (Figure 3) under different scenarios.

3.1. Single Outbreak Site and Single Source of Virus
An early guess and also a dominant view expressed in published reports assumes that SARS-2 outbreak started from a single site in Wuhan, namely, Huanan Seafood Market [46]. However, the only source of bats that have been publicly identified as carrying virus phylogenetically close to SARS-CoV-2 is far away from Wuhan in Zhoushan, Zhejiang. Zhoushan is also one of the largest breeding bases in Zhejiang for bamboo rat, which is suspected as one of the intermediate hosts for SARS-CoV [38,47].
Thus, in order for these bats and/or rats to pass the virus to humans, they must have first been able to migrate or be moved to Wuhan and also must have carried viruses that actually achieved mutations for affording the capability of infecting human beings.
Bats have an ability to migrate more than 1000 kilometers and tend to fly to insect-rich areas [48]. Abundant insects are often found in wildlife market areas due to their selling of various animals. Animal carcasses also make these places suitable habitats for bats. Bats are also attracted to artificial green lights and tend to gather around green light areas [49].
In agreement with these natural characteristics, bats have been found to inhabit locations near Yangtze River Bridge, which has rows of green lights that are tuned on for all of the night-time. Incidentally, Huanan Seafood Market is only 20 minutes away from this bridge. Bats gathered near the Yangtze River Bridge might have released the virus and even infected intermediate hosts for some time. The cold and dry winter helped viruses to survive in the environment and eventually found some ways to cross the species barrier, a phenomenon known as “viral chatter” [50]. The increased vulnerability of human beings in winter time and the increased human exposure to wild animals during holidays made infection to SARS-COV-2 more likely.
At present, there is no evidence to prove the source of bamboo rats in Huanan Seafood Market. Therefore, there are two possible places for bamboo rat be infected with SARS-COV-2.
The first site might be the bat habitat in Zhoushan. Due to the promotion of bamboo rat breeding by Huanong Brothers in 2018, the amount of bamboo rat breeding and market demand increased significantly [51]. Since the market demand increases, the new bamboo rat breeding base may not be far from the local habitat of SARS-COV-2-carrying bats. The model of SARS-COV-2 transmission, similar to Nipah virus, is that farms are built around bat habitats, causing bats to pass the virus to animals through saliva, urine, and feces [30]. At the same time, because Zhejiang is a natural habitat for bamboo rats, it is possible that some farms directly introduced wild bamboo rats, which were already infected with SARS-COV-2 virus. For the above reasons, the bamboo rats carrying SARS-COV-2 virus were transported from the infected place to the incident site in the same way that civets spread SARS-CoV [32].
The second site is Wuhan, the place of the SARS-COV-2 outbreak, and it is also the end point for some bat migration. Zhengli Shi’s team from Wuhan Institute of Virology, Chinese Academy of Sciences, isolated a live SARS-like strain in the feces of horseshoe bats [23]. This suggests that the way the bats spread the virus is not only via direct contact, but also through feces. Therefore, when bats carrying SARS-COV-2 virus forage at Huanan Seafood Market, they may pass the virus directly or indirectly to intermediate hosts.
However, to confirm this scenario, it is necessary to find wild bats in Wuhan and its neighboring areas that carry CoVs identical to those isolated from various SARS-2 patients. It is also necessary to find a mechanism for the very quick outbreak in such a wide area by a natural source of SARS-CoV-2.

3.2. Multiple Outbreak Sites and Multiple Sources of Viruses
Epidemiological investigations showed that 13 of the first 41 patients diagnosed with SARS-CoV-2 had nothing to do with Huanan Seafood Market [45]. Another survey of SARS-2 found that no bats were on sale in Huanan Seafood Market [52].
With so many bats concentrated into a local area, the spreading of viruses by bats might be much wider than just being restricted to one wildlife trading place such as the Huanan Seafood Market. The viruses might have lived in this big “incubation bed” for some time and achieved some mutations before jumping on to the final hosts—human beings.
A study on horseshoe bats in Hong Kong and Guangdong showed that the viruses carried by horseshoe bats in these two places are different. However, some horseshoe bats were found to carry two viruses after mating and foraging activity. This indicates that horseshoe bats not only have the ability to migrate, but also the ability to promote the spread of virus within the same roost and from roost to roost. In addition, sequencing the entire genome of virus carried by bats in multiple regions revealed frequent recombination among different strains. For example, civet SARSr-CoV SZ3 recombination was detected between SARSr-Rh-BatCoV Rp3 from Guangxi, China, and Rf1 from Hubei, China [53]. Therefore, there is a possibility that SARS-CoV-2 spread from Zhoushan to Wuhan due to bat migration.
It turned out that bats are not only attracted by green lights but also red lights [54]. Along the Yangtze River there are also huge bridges decorated with a massive number of red lights. Thus, bats migrating along the Yangtze River might be attracted by these red lights and be relocated nearby. Wuhan might be a new habitation site for a massive number of bats. These bats, coming from different locations, might carry different virus strains. The separate evolution and the recombination of these viruses might lead to the creation of various SARS-CoVs capable of cross-species transmission and ultimate infection of human beings.

3.3. Multiple Outbreak Sites and Single Unique Source of Virus
Many observations have shown the outbreak of SARS-2 actually started from multiple sites, instead of just a single site, as originally reported [27,52,53,55]. In evaluating the epidemiological patterns of SARS-2 within Wuhan, surrounding Wuhan, and remote from Wuhan, it appears that the incidences of SARS-2 have some distinct patterns. Although the remotely occurring SARS-2 usually have a human–human linkage and can be traced to a single source of infection, some Wuhan cases and the surrounding cases in Hubei Province still lack reliable sources of infection. Amazingly, most of the SARS-2 patients can be traced to a single unique etiological agent, SARS-CoV-2. How could this likely single source of virus quickly infect so many people in such large geographic area? This is a question that is difficult to answer now, but must be answered in future.