A man covers his mouth with a handkerchief while walking, ca. 1862

The 21st century is an excellent time to be an infectious disease. Eight billion people now live on Earth, providing countless new hosts; international travel allows for rapid transmission; and climate change is helping some pathogens expand into new areas. As the spread of infectious diseases becomes an ever more challenging threat, it’s crucial to understand the interplay between age — both of hosts and pathogens — and infectious disease.

July 26-28, an interdisciplinary group of researchers gathers at SFI in hopes of deepening that understanding at a three-day working group. Infectious disease expert and theoretical ecologist Mercedes Pascual (University of Chicago), Co-chair of SFI’s Science Board, says scientists still have a lot to learn about the relationship between aging and infectious disease. Pascual is co-organizing the working group with physicist and modeling specialist Jean Carlson of UC Santa Barbara.

“The question of human health is central of course, since we are talking about questions of resilience in the face of aging, and infectious diseases are a major component of human health and what happens with our susceptibility to disease,” Pascual says. “It’s important to our quality of life and the length of our life.”

And just as important as understanding how people respond to infectious diseases at various ages is how those diseases are evolving to outsmart the defenses of their human hosts, she adds. “Pathogens are evolving to escape the immune system,” Pascual says. “The immune system is also adapting to [the pathogen].” HIV and malaria are two examples of this, she notes. “I think we need to better understand how to put those two feedbacks together, and use that understanding to better intervene or control.”

One key topic of discussion will be how the age a person is when she or he is exposed to an infectious disease can affect susceptibility to that disease later in life — especially if the disease has evolved and changed itself.

“I’m talking about pathogens that are escaping our immune system and don’t respond well to vaccines,” Pascual says. “In some sense, two people who are 20 at two different times may have seen something very different in their past. As the virus changes, they may therefore have very different susceptibility.”

The flu is a good example, she adds. “Some ongoing research on the influenza virus would indicate that the types of exposure, the variants of the viruses that we see early in life, determine our responses later on in life. So our immune responses are influenced by the first exposures, the first variants we encounter. It’s the conditions one encounters when young that would then determine later susceptibility. So what matters is what age cohort you are, and not your real age. It’s very interesting and an example of complex time.”

Pascual credits the James S. McDonnell Foundation for supporting research in this area as part of SFI’s Aging, Adaptation, and the Arrow of Time research theme. She says she is hopeful that bringing together experts from fields as varied as immunology, ecology, evolution, and computational science will lead to new insights and further discussion about

the nexus between aging and disease. “A major goal of having this meeting is to bring together people working on different aspects of infectious diseases,” she says. “We have different types of scientists, including let’s say complex systems types that do not work with infectious diseases. The idea is to formulate new questions that are important in this area, and develop new ideas and new models, as well as new ways to look at data.”

Understanding the complex relationships between infectious diseases and aging is crucial in determining how to effectively combat them, adds Carlson. “I believe that the importance of taking a systems level approach is increasingly appreciated, especially as it relates to aging and variability of response to disease and therapies,” she says. “Questions that arise generally in complex systems — adaptation, co-adaptation (of the host and the pathogen), complex and time-varying environments, diversity, dynamics, tradeoffs, information, and entropy — are central to understanding infectious diseases and developing robust therapies.

Read more about the Aging and Adaptation in Infectious Disease working group.