It could take just one cough, one kiss, one touch or even one bite to change not only your life, but the lives of everyone around you -- and for months or even years.
In most cases, the closer those people are to you, the greater the risk. But it isn't always that simple.
The risk at hand: an infectious outbreak.
Public health experts believe we are at greater risk than ever of experiencing large-scale outbreaks and global pandemics like those we've seen before: SARS, swine flu, Ebola and Zika.
More than 28,000 people were infected during the 2014-16 Ebola epidemic, with over 11,000 deaths. And as of March 10, 84 countries have reported Zika transmission. That disease was discovered in the 1940s, but had its first outbreak in 2007 in Micronesia, and more recently began spreading toward the end of 2015.
Every time, the infection's arrival is unexpected and its scale unprecedented, leaving the world vulnerable.
Experts are unanimous in the belief that the next outbreak contender will most likely be a surprise -- and we need to be ready.
"We're only as secure in the world as the weakest country," said Jimmy Whitworth, professor of international public health at the London School of Hygiene & Tropical Medicine. With so many health systems and economies in a fragile state, this means we are far from secure.
"Infectious diseases respect no boundaries," he said. The World Health Organization is alerted to hundreds of small outbreaks every month, he noted, which it investigates and uses to predict the chances of a bigger problem.
"There are little clusters of outbreaks occurring all the time, all over the place," Whitworth said.
But with infections disregarding borders and their battle lines against humans drawn, he believes the way we live today is what opens us up to risk.
"(Many) aspects of modern life put us at more risk. We are more ready than before," he points out, highlighting the International Health Regulations Global Outbreak Alert and Response Network and countries with national rapid response teams -- such as the United States, UK and China -- ready to tackle any emergency.
"But the stakes keep getting raised," he said. Here's why.
1. Growing populations and urbanization
The facts around urban living are simple: You live, eat, work and move closer to people than in any rural setting, and with this comes greater opportunity for disease to spread through air, mosquitoes or unclean water.
As populations grow, so will the number of city-dwellers, with the United Nations predicting that 66% of the global population will live in urban areas by 2050.
More people in cities can "put a strain on sanitation," said David Heymann, head of the Centre for Global Health Security at the think tank Chatham House. Beyond people's close proximity, "this is a second source of infection," he said, and a third is increased food demand, causing farmers to grow more food, with more animals, making them likely to live closer to those animals as well.
Animals are reservoirs for many diseases, including cattle for tuberculosis and African sleeping sickness (trypanosomiasis) and poultry for avian flu.
With people moving more regularly from -- and between -- rural settings to urban ones, the chances of them becoming infected and then living in close quarters with others further boosts the potential for things to spread.
2. Encroaching into new environments
As numbers of people grow, so does the amount of land needed to house them. Populations expand into previously uninhabited territories, such as forests. With new territories comes contact with new animals and, inevitably, new infections.
For one example, "Lassa fever occurs because people live in the forest and destroy it for farming," Heymann said.
Lassa fever is a viral disease spread by contact with the feces of infected rodents. It can cause fever and hemorrhaging of various parts of the body, including the eyes and nose. Person-to-person transmission is also possible, albeit less common. Outbreaks generally occur in West Africa, with higher than expected rates in Nigeria since 2016.
Heymann explains that Lassa is one example of people living near forest environments where infected rodents reside, but destruction of those forests for agriculture leaves the animals nowhere to go -- other than humans' homes.
"The rodents that live there can't get food and go into human areas for food," he said.
3. Climate change
Evidence continues to emerge that climate change is resulting in greater numbers of heat waves and flooding events, bringing more opportunity for waterborne diseases such as cholera and for disease vectors such as mosquitoes in new regions.
"Flooding is occurring with increased frequency," Heymann said, and with that comes greater risk of outbreaks.
Between 2030 and 2050, climate change is projected to cause about 250,000 additional deaths per year from heat stress, malnutrition and the spread of infectious diseases like malaria, according to the World Health Organization.
With disease carriers like mosquitoes increasingly able to live in new unprotected territory, the risk of an outbreak is high.
Whitworth cited the current yellow fever outbreak in Angola, which has infected more than 350 people. He explained that as workers from China returned home from Angola, any yellow fever infection could have been transmitted by mosquitoes in China.
But, the workers' return in winter meant the insects weren't around to transmit through bites.
4. Global travel
"We're vulnerable because of increased travel," Whitworth said.
International tourist arrivals reached a record of almost 1.2 billion in 2015, according to the UN World Tourism Organization, 50 million more than 2014. It was the sixth consecutive year of above-average growth. And with greater numbers moving at all times come greater options for infections to hop a ride.
"Infectious agents travel around in humans many times within their incubation period," Heymann said. An incubation period is the time between infection and the onset of symptoms, meaning people can transmit an infection though they won't appear to be sick.
The SARS (severe acute respiratory syndrome) pandemic of 2003 is thought to have begun with Dr. Liu Jianlun, who developed symptoms of the airborne virus on a trip to Huang Xingchu in China and then went to visit family in Hong Kong. He infected people at his hotel and his family. He was then hospitalized and died, as did one of his relatives.
In less than four months, about 4,000 cases and 550 deaths from SARS could be traced to Liu's stay in Hong Kong. More than 8,000 other people became infected across more than 30 countries worldwide.
But Heymann stresses that "it's not just humans" who spread disease through travel. Infections spread through insects, food and animals moved between countries. "It's also trade," he said, pointing to airport malaria, in which people in airports have become infected with malaria through mosquitoes that have hitched a ride on a plane or in food.
He also described bird flu that was stopped at the Belgian border in Thai eagles being traded as pets in 2004. Guinea rats shipped as pets in the United States in 2003 harbored infections with monkey pox, he noted, which then entered prairie dogs and eventually humans.
5. Civil conflict
"If a health system cannot handle (an outbreak), there's pandemonium," Heymann said. He believes that poor hygiene is not a valid excuse anywhere, even in developing settings, as sterilization and hand-washing are straightforward.
But if a country is on the brink of breakdown from civil unrest, the ability to handle an intense and sudden problem like an outbreak could bring its people to their knees -- and allow the infection to flourish.
"Outbreaks can completely paralyze countries," said Whitworth, citing the 2014 Ebola epidemic in which Sierra Leone, Guinea and Liberia were "quite close to collapsing."
Civil unrest had plagued all three countries, leaving their economic and health infrastructures in dire need of rebuilding -- and ill-prepared for a major infection to strike.
That problem combined with human movement between these three countries and others more globally meant Ebola was able to spread, even though dozens of infections in previous years in nearby Democratic Republic of Congo were self-contained and often resolved themselves.
"If (an infection) stays local, it burns out," Heymann said. "People learn what to do."
6. Fewer doctors and nurses in outbreak regions
Beyond weak health systems, countries where outbreaks are more likely to occur -- namely, more developing settings -- also have fewer doctors and nurses to treat the population. Most have left for better prospects elsewhere.
"We have to deal with that as a reality," Heymann said, adding that some countries even encourage young medical talent to travel to new regions.
"It's difficult to manage health worker migration," he said. But programs and strategies are underway to tackle this by "task-shifting," moving responsibilities to new groups and training them to deliver care, such as community health workers.
"Communities have to be resilient," he said, and assigning tasks to people at all levels could mean a greater team available when a new infection strikes.
7. Faster information
In the information age, new levels of communication bring even newer levels of fear and multiple ways to spread it, experts believe.
Although the majority of small outbreaks may once have gone largely unknown by populations farther from the epicenter, people today are more informed than ever and require transparent, factual information to be fast-flowing.
Google has been been using searches for symptoms to help identify when an outbreak may occur, such as with the flu.
"The world looks for an authority," said Heymann, who believes the WHO adopts that role but needs to be faster and more transparent with information. The organization was criticized for being too slow to respond and unprepared for the 2014 Ebola outbreak.
"But social media has become active ... and that's an area that's difficult to control," he said.
The posting and shaping of information by multiple people can change messaging and what people read and believe, Heymann added. It may not all be bad, he said, but the point is that it can shape the way information travels, potentially inciting fear and stigma.
"Not all information on the internet or social media is accurate," said Mark Feinberg, chairman of the scientific advisory committee of the recently launched Coalition for Epidemic Preparedness Innovations. "Ensuring accurate communication to the public is critically important."
The coalition, launched in January, will address the surprise nature of outbreaks and epidemics to try to prevent them, rather than respond to them.
Lining up the elements
Heymann describes the likelihood of a new infection spreading rapidly and becoming an epidemic -- and potentially a global pandemic -- using the analogy of lining up pieces of Swiss cheese, with the different risk factors equating to holes in the cheese. "When they line up, you get an epidemic," he said.
He highlighted an outbreak of Rift Valley virus in East Africa in 1997. The combination of an El Niño event pushing humans away from their homes and closer to cattle, combined with increased rainfall producing more breeding sites for disease-spreading mosquitoes, led to the largest documented outbreak of this virus. It involved five countries and infected an estimated 90,000 people.
"All these (factors) came together and led to an outbreak," Heymann said.
Despite what we know about the aspects of modern life that put us at greater risk, all three experts believe the world is not quite ready to handle what is inevitably coming.
"We need to do a lot better," Feinberg said. "We need to prepare in advance, not respond."
His program, the Coalition for Epidemic Preparedness Innovations, is working to do just that. It's aiding the development of vaccines against viruses that it believes need attention and are in families of infections that are likely to pose a risk, such as MERS CoV, which continues to persist in the Middle East and has been reported in almost 20 countries outside that region.
The program will also be looking to develop platforms on which vaccines can be made more rapidly so that the general development time frame of 15 to 20 years can be shrunk significantly to respond to a new virus -- even moreso than those being developed against Ebola and Zika.
"That's the kind of capability we need," Feinberg said. "The pathogens we don't know about pose the greatest threat."
Combined with other global and national strategies programs -- such as WHO regulations and national response teams -- Feinberg is optimistic.
"We are far away from that goal," he said. "But I am encouraged, as they are all working on this