The attack rate or transmissibility (how rapidly the disease spreads) of a virus is indicated by its reproductive number (Ro, pronounced R-nought or r-zero), which represents the average number of people to which a single infected person will transmit the virus.
WHO’s estimated (on Jan. 23) Ro to be between 1.4 and 2.5. 
Other studies have estimated a Ro between 3.6 and 4.0, and between 2.24 to 3.58. .
An outbreak with a reproductive number of below 1 will gradually disappear.
For comparison, the Ro for the common flu is 1.3 and for SARS it was 2.0.