The fluid dynamics of disease transmission:

In studying the spread of infectious disease, Lydia Bourouiba questions everything. This includes the accepted WHO classification of droplet-based contagion, as defined some 80 years ago.

“There’s a dichotomy that plagues how we see transmission right now—large droplets versus aerosols—because it’s way too simplistic and could be deceiving,” says Bourouiba.

In the 1930s, seminal researcher William Wells introduced the concept of suspended small droplets, still considered the respirable villains in aerosol transmission.

“Yet his research—this accepted dichotomy of large versus small droplets—doesn’t tell the whole story. Sneezes and coughs are actually a violent expiration full of complex physics and fluid dynamics that are key to the range of contamination,” says Bourouiba. “Each sneeze contains a whole size spectrum of pathogen-containing droplets and a gas phase that can enhance range and evaporation. Also, the ambient moisture and air temperature can strongly change the cloud dynamics and the size and absolute value of droplets can shift. In other words, a droplet in one ambient may quickly settle, while in another it would be carried away.”

All this results in a soup of variables almost too complex to fathom. Yet through high-speed visualization methods (pioneered at MIT by Harold “Doc” Edgerton), fluid dynamics experiments and theoretical models, Bourouiba’s group has gained new insights into the reach of a sneeze and its turbulent cloud. And a new window into the in-host dynamics of infection.

“To see those high-speed images was amazing, because I could see that if we tackle this properly, it could root epidemiology in fundamental physical mechanisms, not just phenomenological models and statistics. It could finally help link breakthroughs in immunology to human-to-human transmission and population-level epidemic,” says Bourouiba.

“There are dramatic headways in immunology, genomics and microbiology, but they cannot answer the overall patterns of outbreak. What’s going on at the scale of an epidemic? What’s the probability of transmission between people in this particular room or set-up or building?”

Such inquiry leads to more complex questions about what can make a disease more or less infectious. And under what conditions.

“Think about Ebola in Western Africa in 80% relative humidity and high temperature versus a similar outbreak in a drier environment. The environmental conditions would change the range of the aerosol cloud and its payload, as well as what happens to the droplets and their pathogens,” she says.

By methodically tackling questions of pathogen transport, the spread of many infectious diseases could be unlocked. Other areas of Bourouiba’s focus include: communal diseases from contaminated water in hospitals, enrichment of pathogens through bubble-bursting, the breakup of mucosalivary fluid that creates droplets—and even the spread of agricultural pathogens through proximity, rain and fluid-based processing.

“We can ask: are pathogens passively directed by fluid mechanics or do they affect surrounding fluids to their advantage?” Bourouiba says. “These are multiscale complex problems that we are only starting to examine closely with the quantitative and mechanistically inquisitive approaches of physical mathematics and fluid dynamics.”

Bourouiba hopes to help pave that path.

“Colleagues in IMES & CEE could start to think about applications—new indoor airflow and ventilation, new materials, optimal spacing, new protective gear, redesigning confined places like airplanes, as well as updating the foundation of predictive epidemic models and epidemic risk assessments. All to intervene in a smarter, more targeted way at the onset of new outbreaks.”

Joining MIT in 2010 as an NSERC (Natural Science and Engineering Research Council of Canada) post-doctoral fellow in Applied Mathematics—and now the Esther and Harold E. Edgerton Assistant Professor in the Department of Civil and Environmental Engineering (CEE) and IMES—Bourouiba says, “MIT is a very stimulating place. Working with students who are as excited about these problems as me is absolutely thrilling. MIT is the place where I could pursue this. The students, the energy, the curiosity and openness, the fearless questioning—here, one can pursue a vision and make it a reality.”

Bourouiba’s ultimate vision roots epidemiology and public health policy firmly in the tools, methods, insights and mechanistic knowledge honed over centuries in the physical and mathematical sciences.

“Epidemics have shaped human history throughout time. Today we want to be sure that the full depth of understanding from all corners of science are leveraged to mitigate epidemics and save lives.”