What determines if a disease will spread like wild fire? The rate of transmission

COVID-19 is thought to spread mainly through close contact from person-to-person in respiratory droplets from someone who is infected, and showing symptoms of the illness. However, some people without symptoms may be able to spread the virus.
What determines if a disease will spread like wild fire, is how many patients does one individual infect.
According to the U.S. Centers for Disease Control and Prevention, how easily a virus spreads from person-to-person can vary.
Information from the ongoing COVID-19 pandemic suggests that the virus is spreading more efficiently than influenza.
The University of Minnesota's Center for Infectious Disease Research and Policy says a recent study that focused on the first 425 cases in China suggested that the virus has a basic reproductive value of 2.2, meaning that each patient could spread the virus to about two other people. These results, however, were preliminary. Factors like quarantines and stay-at-home orders influence the spread of a disease.
"If it is one, where one person infects only one additional person or less, then the disease is usually stable. However, if one person infects two people, and each of those two people infects another two people, it becomes logarithmic, and it just takes off," says Dr. Adam Singer, the vice chairman for research in the Department of Emergency Medicine at Stony Brook University Hospital.
Dr. Singer says that the way to keep infections low, is to practice social distancing and wear masks.
According to Gov. Andrew Cuomo, in New York, after over a month of stay-at-home orders, the current statewide figure is 0.8, meaning 10 positive people are infecting about eight others.
Gov. Cuomo says to keep coronavirus cases low, the transmission rate should not go over 1.2.