Last week, the World Health Organization convened a special committee to decide whether to declare a global health emergency. Ultimately, after two days of deliberation, the group decided not to take this major step, though the W.H.O.’s director general, Dr. Tedros Adhanom Ghebreyesus, noted ominously that “it may yet become one.” That time may have come: Today, the W.H.O. committee will convene again to determine if the outbreak has crossed a threshold into warranting a designation of a global emergency. As it considers how to proceed, the W.H.O. committee is grappling with a monumental choice — global health emergencies are legally binding announcements requiring every United Nations member state to take action — with scant data.

I don’t doubt that the epidemic triangle is critical in helping W.H.O. navigate this process. Is this outbreak because of a substantive shift in the genetic makeup of an old pathogen, a change in the vulnerability of the host population or an alteration of the environment that has shaped the threat emanating from Wuhan? As the Wuhan outbreak continues to expand, applying the calculus of the epidemic triangle remains the most reliable way to chart an effective public health response while avoiding a descent into hysteria.

First, we need to know how much of a shift this new pathogen is from previous coronavirus strains, based on its infective potential and virulence. Judging from the rapid rise in cases since the first cluster was reported on Dec. 31, the new Wuhan strain could very well be highly infective. For perspective, SARS has a relatively low basic reproductive ratio (or R0) of 0.5, meaning that every two cases of SARS results in only one additional infection. Very early estimates shared by the W.H.O. suggest the Wuhan strain’s R0 is 1.4 to 2.5, producing roughly two secondary cases for each initial infection, which places it slightly higher than the seasonal flu (for perspective, measles has a R0 of 12 to 18).

If the new coronavirus’s R0 remains low as more data become available, we can be relatively confident that surveillance and quarantine will stem its spread, though these measures become steadily less effective as the R0 increases. With respect to its virulence, early genomic sequencing of the Wuhan virus suggests, thankfully, that it is only distantly related to SARS (they are only 73 percent identical), meaning that it is likely to be less deadly, though it is too early to say with confidence.