With every disease outbreak, researchers try to figure out how far — and how fast — a virus is likely to spread through a population.
To do that, they use a number called an "R naught," or R0. Most simply, the figure refers to how many other people one sick person is likely to infect on average in a group that's susceptible to the disease.
The number is super important in the context of public health because it foretells how big and deadly an outbreak will be. The higher the number, the greater likelihood a lot of people will fall sick. Measles, the most contagious virus researchers know about, can linger in the air of a room and sicken people up to two hours after an infected person who coughed or sneezed there has left. If people exposed to the virus aren't vaccinated, measles' R0 can be as high as 18.
Ebola is more deadly but much less efficient: Its R0 is typically just 2, in part, because many infected individuals pass away before they can pass the virus to someone else.
Zika falls somewhere in between. Researchers just came out with an early estimate for Zika's R0 in the current outbreak based on data from Colombia, and they've guessed it falls between 3 and 6.6.
The true R0 ultimately depends a lot on the context
There's one important thing to remember: The R0 for a disease is never final. The number can change with every outbreak, and even the R0 for individual viruses can vary wildly depending on the context.
As infectious diseases epidemiologist David Fisman explained, the factors that go into determining the R0 include how many contacts a sick person is likely to make, the number of transmissions that are possible per contact, and how long those people shed the virus and potentially infect others.
"Any one of these factors is going to be influenced by social norms, how people interact, how much infrastructure — like sewage treatment or clean water — is available in a place, and how cases are managed," Fisman said. The number is also influenced by population density, vaccination levels, and access to treatments.
For a pathogen like norovirus — that nasty and highly contagious bug infamous for causing outbreaks of stomach flu on cruise ships — the R0 estimates vary depending on whether the outbreak is contained in one place (like a hospital) or spread more widely.
For mosquito-borne diseases like Zika, dengue, and chikungunya, there are additional complications: The R0 can be change depending on the presence of mosquitoes, population density, and whether people live in air-conditioned environments where bugs are less likely to bite.
That's why Isaac Bogoch, an infectious diseases doctor, noted that if Zika spreads in the US, the R0 for an outbreak will probably be lower than in Latin America.
"Even if the mosquito species is there, are they there in high or low numbers? Is the environment even conducive for mosquitoes to transmit infections?" Bogoch asked, adding that the answers to these questions will vary by state since there are more Aedes aegypti (the mosquitoes that transmit Zika) in the southern US than in the north.
But with a relatively little-understood pathogen like Zika that's never spread here before, only time will tell.