The coronavirus is easy to transmit and difficult to detect, a combination which makes its spread across the globe highly likely. COVID-19 spreads primarily through respiratory droplets, small drops of fluid which are released when an infected person, coughs, sneezes, or speaks. Infection occurs when respiratory droplets containing active coronavirus land on an uninfected person's face or are transferred there by the person touching their face with their hands. No vaccine or treatment for COVID-19 is currently available, so when an uninfected person encounters the coronavirus, there is nothing to prevent the infection from taking hold and running its course. Transmission of the coronavirus is made more difficult to avoid by the fact that it is hard to know who is infected. Typical symptoms of COVID-19 mirror symptoms of the common cold and flu, and symptoms can take up to two weeks to appear after exposure to virus. Some people may not experience symptoms at all. Without a reliable means of early detection, quarantine and social isolation cannot be effective containment strategies, because it cannot be determined who needs to be isolated in sufficient time to prevent further transmission. Epidemiologists characterize the transmissibility of a disease using the basic reproduction number, which is the number of other people each infected person is expected to spread the disease to, on average. COVID-19's number is estimated at between 2 and 3, which would make it sufficiently transmissible to spread through a large portion of the population if left unchecked.