Introduction
In December 2019, an outbreak of unidentified pneumonia characterized by fever, dry cough, and fatigue happened in Wuhan, Hubei, China (Zhu et al., 2020). With the spread of the disease, the number of infected patients increased substantially which has become the most challenging health emergency all over the world. Sequence analysis of the coronavirus has shown a structure typical to that of other coronaviruses such as SARS coronavirus and MERS coronavirus (N. Chen et al., 2020). It also revealed that the new coronavirus has the smallest genetic distance from bat coronavirus, and about 80% similarity with SARS-CoV, and 50% similarity with MERS-CoV (P. Sun, Lu, Xu, Sun, & Pan, 2020). Thereafter, the coronavirus was designated as SARS-CoV-2 and the infectious disease was named Coronavirus Disease 2019 (COVID-19) by the World Health Organization (WHO) (Li-Li et al., 2020; Sohrabi et al., 2020; Zhu et al., 2020). Besides, the nosocomial transmission was detected on January 20, 2020, which suggested that COVID-19 can be transmitted from human to human.
Due to the effective prevention and control measures, the epidemic has been brought under control in China, South Korea, Japan, and more than 90% of patients have recovered and discharged in these countries. However, the confirmed cases are still growing rapidly in American, European and African countries including the United States, Brazil, Russia, the UK and South Africa (WHO, 2020a). As experience in recognition, diagnosis, and treatment of COVID-19 builds, WHO has launched global megatrial of the four most promising coronavirus treatments (WHO, 2020b). The present article is to provide a review of the characteristics of COVID19, including the epidemiology, etiology, clinical features, pathological changes, and treatment, and the experience of prevention measures for this disease.
1. Epidemiology ofCOVID-19
Since December 2019, the first 27 cases of unidentified pneumonia were reported by the Wuhan Municipal Health Commission (Ashour, Elkhatib, Rahman, & Elshabrawy, 2020). On January 11, 2020, the pathogen of the pneumonia was initially confirmed as a novel coronavirus. On January 20, 4 confirmed cases of COVID-19 were reported from three countries outside of China including Thailand (2 cases), Japan (1 case), and the Republic of Korea (1 case). As the epidemic expanded, daily confirmed cases increased to 14,109, before falling to 1748 on February 18. And since then, the number of daily emerging cases gradually falls below 1000 for the first time in China. However, the number of confirmed cases reported daily increased rapidly abroad. Data released on February 25, 2020 showed foreign countries had overtaken China in confirmed cases per day for the first time. And then the disease spread rapidly in Europe and in the United States (Figure 1a) . Up to this point, there have been 84,543 confirmed cases in China and 6,218,350 cases were confirmed in 212 countries outside of China with 370,100 fatalities(Figure 1b) . Recently, the five countries with the greatest number of patients analyzed were the United States (1,839,679 cases, 106,261 deaths, case-fatality rate: 5.8%), Brazil (514,992 cases, 29,341 deaths, case-fatality rate: 5.7%), Russia (414,878 cases, 4855 deaths, case-fatality rate: 1.03%), Spain (286,509 cases, 27,127 deaths, case-fatality rate: 9.5%), and the United Kingdom (274,762 cases, 38,489 deaths, case-fatality rate: 14.0%) (WHO, 2020a) (Figure 1c) . Since the first case of COVID-19 was reported in the U.S., the numbers of confirmed cases and deaths increase exponentially with steady growth at 20,000-30,000 per day. Though the outbreak of COVID-19 appears at later stage in Russia and Brazil, the number of confirmed cases has exceeded most European and American countries. From November 2019 to May 25, 2020, the number of cumulative deaths caused by the COVID-19 was 350,057, and the overall case-fatality rate (CFR) was 6.3% which was lower than that of the SARS (9.60%) and MERS (34.4%) (WHO, 2020a)(Figure 1d, Table 1) .
The transmission of infectious diseases must rely on three requirement conditions: sources of infection, routes of transmission, and susceptible hosts. A growing body of scientific evidence suggests that COVID-19 is a zoonotic disease as with SARS and MERS, and originated from the wild bat (Evans, 2013; Keeling & Rohani, 2011). Though the strands of evidence suggest the possibility of an initial zoonotic emergence, the role of intermediate hosts such as pangolins, snakes, turtles, hamster, ferret, and other wild animals in the origin of SARS-CoV-2 still remains controversy (Tiwari et al., 2020). Close contact with symptomatic cases and asymptomatic cases with silent infection are the main transmission routes of 2019-nCoV infection. It suggested that SARS-CoV-2 can be transmitted through respiratory aspirates, droplets, contacts, and digestive tract transmission that remained to be confirmed (Peng et al., 2020). Vertical transmission was sporadically reported in some media but not yet proved(H. Chen et al., 2020). Reports showed that the basic reproductive values (R0) of COVID-19 were calculated between 1.5 and 3.5 (Eisenberg, 2020; Natsuko Imai, 2020). Therefore, SARS-CoV-2 appears to be more infectious than SARS-CoV or MERS-CoV based on R0 values at the early stage of this outbreak (Ying, A, Annelies, & Joacim, 2020) (Table 1. ). Similar to SARS and MERS, nosocomial transmission was a severe problem. COVID-19 has posed a difficult challenge to healthcare facilities from both the impact of healthcare-associated transmission and the resource burden of controlling and preventing further spread. It has been reported that a total of 3,019 health workers were infected, accounting for 4.17% of total cases (Epidemiology Working Group for Ncip Epidemic Response & Prevention, 2020). In terms of susceptible populations, people are generally susceptible to COVID-19 regardless of age or gender. And the elderly and those with underlying chronic diseases (hypertension, diabetes, COPD) are more likely to become severe cases. A retrospective cohort study by Chao et al. also demonstrated that patients with hypertension and diabetes accounted for 20-30% of total infected patients and had a higher risk of developing into acute respiratory distress syndrome (ARDS) or multiple organ failure, and have a higher case fatality rate than those without (Chaomin, 2020). The ACE2 receptor, an important member of the renin-angiotensin system (RAS), is highly expressed in the cardiovascular/cerebrovascular and lung tissue in the hypertension patients (Kristensen et al., 2015). Current reported data revealed that 86.6% of confirmed patients were aged 30-79 years and 1-2% of the patients were children and newborns (1.5 months to 17 years). The clinical course of pregnant women with COVID-19 is similar to patients of the same age (Schmid & Fontijn, 2020).
2.The etiology of COVID-19
Coronavirus is comprised of single-stranded positive RNA virus that belongs to an order Nidovirales , familyCoronaviridae , and subfamilyOrthocoronavirinae (Jie, Fang, & Zheng-Li, 2019). Coronavirus can be divided into four genera: α-, β-, γ-, δ-coronavirus according to the characteristics of serotype and genome (P, Xin, P, & Y, 2019). Genome sequences analysis showed that the novel coronavirus is a new type of coronavirus (SARS-CoV-2) and belongs to the β-CoV genera (Li-Li et al., 2020; Ren et al., 2020). Genetics researchers and scientists worldwide found that the genomes of SARS-CoV-2 have naturally evolved, thereby rejecting a hypothesis of a possibility of recent recombination event as the cause of this outbreak (X. Xu et al., 2020). Since its genome is similar to bat coronavirus, scientists conjecture SARS-CoV-2 is most likely to be originating from the bat (Cui, Li, & Shi; Rothan & Byrareddy, 2020). Virion spike (S) protein is the key components which determines the host tropism of the virus. S protein is composed of two subunits: S1 and S2, which are responsible for the receptor binding and fusion of virus with cell membrane respectively (Shajahan, Supekar, Gleinich, & Azadi, 2020). Current studies have revealed that SARS-CoV-2 shared the same receptors with SARS-CoV and MERS-CoV for invading the host cells, and the angiotensin-converting enzyme 2 (ACE2) receptor has been identified as the main cell entry receptor of ARS-CoV-2 (Kuba et al., 2020). And structural and biophysical analysis revealed that the affinity of SARS-CoV-2 for ACE2 is 10 to 20 times higher than that of the SARS-CoV (Xie & Chen, 2020). A vitro study confirmed that SARS-CoV-2 can survive in human respiratory epithelial cells for 96 hours in vitro (Yu, 2020). SARS-CoV-2 also shared the same physical and chemical characteristics with SARS-CoV and MERS-CoV. Coronaviruses are hypersensitive to ultraviolet rays and heat. And it can be killed easily by exposed to 56 ℃ for 30 mins, 75% ethanol, chlorine disinfectant, peracetic acid, and chloroform (Deng & Peng, 2020).
3. Clinical characteristics ofCOVID-19