Coronaviruses are a large family of viruses in the natural world. They have small sizes, only 100 nanometers or so in diameter, and can only be visible under the electron microscopes. A coronavirus has a roughly spherical shape and has many protuberances on its surface. Because of their characteristic crown-like appearance, the coronavirus is named after corona, which means any structure that resembles a crown in shape. Coronavirus is an RNA virus, which has one single outer membrane enveloping several genes. It is easy to mutate because its genetic material is a single-stranded RNA.

There are 15 types of different coronaviruses that can infect mammals such as mice, dogs, cats, wolves, horses, pigs and cows, as well as chickens and other birds.There is a cluster of members in the coronavirus family. Only six kinds of coronaviruses infecting human beings had been found before December 2019. Four of them make people mild symptoms of common cold, and generally recover within two weeks. But the other two are violent infections, once infected, they can cause serious respiratory diseases. One is named severe acute respiratory syndrome virus (SARS-CoV), which caused the SARS epidemic in China in 2003; the other one is named Middle East respiratory syndrome virus (MERSCoV), which was first found in the Middle East in 2012.

At the end of 2019, a novelcoronavirus (had never been found in humans before) was identified as the cause of a cluster of pneumonia in Wuhan, the provincial capital of Hubei China. It rapidly spreads, resulting in an epidemic throughout China and a threat of global pandemics. The World Health Organization (WHO) designates this new virus as 2019- nCoV and the disease as COVID-19 that belongs to the family of coronavirus.

The novel coronavirus’ transmission mechanisms is incomplete currently. Studies show the spread of 2019- nCoV may be similar to SARS-CoV whose hosts are bats. And the routes of transmission are likely to be the same: they are likely transmitted among birds and mammals, with bats being host to the largest variety of genotypes. Finally, human beings become the infectors after touching these wild animals carrying the virus.

People are generally susceptible to 2019-nCoV. The elderly and those with basic diseases are seriously ill after exposure, and there is evidence to children and babies infected with 2019-nCoV. Pneumonia caused by the novel coronavirus can occur in immune compromised and immune competent population, which is related to the amount of virus exposed. In case of a lot of viruses are exposed at once, there is a high risk of its infection even if people have normal immune function, which means whether infected mainly depends on the chance of contact. In the same exposure opportunities, the elderly, people with chronic diseases or people with abnormal immune function are susceptible to be infected.

The incubation period for 2019-nCoV is thought to be within 14 days following exposure, with most cases occurring approximately 3-7 days after exposure. It may also be contagious during the incubation period.

Theoretically, one patient can transmit the virus to 2-3 individuals without safety precautions. Person-to-person spread is thought to occur mainly via respiratory droplets and contact. However, given the current uncertainty regarding transmission mechanisms, spreads via air and stool specimens remain unknown. Respiratory droplets transmission generally, face-to-face, talking, coughing, and sneezing can cause droplets to spread. Therefore, wearing a mask seems an effective prevention, as well as keep your distance from others as much as possible (at least 1-2 m).

Direct contact transmission refers to the transmission of the virus through direct contact of mucous membrane or skin.

Indirect contact transmission refers to the transmission caused by:
Touching contaminated objects, feces (such as stool, which has been still unsure),secretions (such as droplets accompanied by coughing and sneezing), or daily necessities (learning supplies, bedding, toys, tableware, clothing etc.), hands and the contaminated hands touch the nose, mouth and eyes and mucous membranes (therefore, washing your hands frequently is the most important precautionary measure).

Common cold generally refers to upper respiratory tract infections that mainly manifest nasopharyngeal symptoms caused by cold, fatigue, etc. It's a common acute upper respiratory tract viral infectious disease. Symptoms of colds are mainly nasal congestion, runny nose, sneezing, no obvious fever, no obvious changes in physical strength or appetite.