On Monday, the World Health Organization (WHO) declared the Zika virus a public health emergency of international concern, with the potential to infect up to four million people in the Americas by year’s end. Under increasing pressure to slow the spread of the disease, the Brazilian government has stepped up its response, including mobilizing 220,000 military personnel to cities across the country.

According to Brazil’s ministry of health, the military will be used to spread awareness by going door-to-door, handling out pamphlets, and distributing mosquito repellent. However, it is believed that the troops will also use this opportunity to identify which homes are potential breeding grounds for mosquitoes and target those sites for surveillance and fumigation. Mosquitoes are known to carry the Zika virus, which has been linked with the rare congenital condition microcephaly in newborn babies.

In the United States, meanwhile, the Pentagon announced that it will collaborate with the Department of Health and Human Services to help contain the Zika outbreak. The Department of Defense is active in efforts to contain mosquito-borne diseases as part of ongoing tropical disease research through agencies such as the Army Medical Research Institute of Infectious Disease and the Defense Threat Reduction Agency.

This isn’t the first time in recent history that non-civilian actors have been called upon to assist in public health emergencies. In 2014, foreign militaries deployed more than 5,000 personnel to West Africa as part of the international Ebola response. Additionally, the Pakistani army is playing a key role in facilitating poliovirus vaccinations in that country’s volatile tribal regions. Even the language used is noticeably confrontational, with Brazilian officials speaking of the “battle against Zika,” which is the country’s “number one enemy.” Does this indicate a trend towards increasing militarization of public health emergencies? If so, what implications does this have for future health crises?

The involvement of foreign military during the Ebola crisis was clearly unprecedented. According to a recent University of Sydney report on civil-military relations during that outbreak, the US military under Operation United Assistance deployed approximately 2,900 personnel primarily to Liberia to train local health workers and set up treatment units. Under its Operation Gritrock, the United Kingdom deployed some 750 military personnel primarily to Sierra Leone beginning in October 2014. Other countries’ militaries such as Canada and Germany also contributed.

While some military personnel provided direct patient care, they principally served a coordination and logistics function, including providing key assets such as military aircraft, personal protective equipment, and beds. They also supported the domestic militaries of the affected countries, which were themselves mobilized against Ebola by staffing checkpoints, enforcing quarantines, and assisting with safe burial practices. Foreign military assistance was integral in stopping the spread of the virus, and it likely would not have been contained without it, according to the report.

In addition, the Pakistani Army is being called upon to provide security for polio vaccination workers in rural Pakistan, one of only two countries in the world where poliovirus is still active. Efforts to eradicate the virus faced setbacks after Taliban forces began deliberately targeting vaccination workers, most recently on January 13^th, when a suicide bombing killed 15 people at an immunization drive. As of January 2015, the Pakistani Army began regularly escorting vaccination workers and providing them with visible security. This has contributed to a 70% drop in new polio cases over 2015.

Devi Sridhar, Professor of Global Public Health at the University of Edinburgh Medical School, told me that the outbreaks of poliovirus, Ebola, and Zika have done much to highlight the link between health and the military. However, the association is not new. For instance, many Western militaries have sought to promote health causes for years. “The US Department of Defense is one of the top five biggest funders of research and development for diseases that primarily affect low and middle income countries,” Sridhar said.

A 2013 Kaiser Family Foundation report by Kellie Moss and Josh Michaud analyzed the Pentagon’s considerable contributions to global health and infectious disease efforts. The report states that the military places a “high priority” on protecting personnel from exotic diseases in order to maintain the fitness and operational readiness of the force. While much of this research is carried out by civilian laboratories through grants, the military also runs its own scientific research labs, such as the Naval Medical Research Center.

A top priority is research and development for HIV/AIDS and malaria prevention and treatment. The US Army received $23 million USD for malaria-related funding in 2013 alone. Other areas include designing countermeasures for bioterrorism and improving understanding of post-traumatic stress disorder. More recently, the Department of Defense was able to accelerate Ebola vaccine research. In 2014, Ebola research and development received $11 million USD from the Pentagon’s Defense Threat Reduction Agency.

Adam Kamradt-Scott, Associate Professor at the University of Sydney and lead author of the report previously mentioned, told me that while the Ebola outbreak was exceptional for the participation of foreign militaries, evidence suggests that domestic militaries have regularly been active in containing public health crises. This is particularly true of developing countries, where the military is often well-respected and viewed as more competent and less corrupt than other government institutions.

“In resource-poor countries, militaries are often called upon to assist in all sorts of crises, from a simple search and rescue operation to health surveillance and even healthcare,” Kamradt-Scott said. “In several countries throughout Africa and Asia, militaries are readily called upon to assist in activities that Western sensibilities would find unusual and even objectionable.”

He said he suspects that Brazil decided to deploy its military because it is a large, competent, and professional force that can be expected to follow orders and effectively carry out its mission.

According to Kamradt-Scott, the examples of recent disease outbreaks underscore the lack of research on links between health and the military, as well as the absence of clear rules and guidelines governing military involvement in health emergencies. “We need to understand more fully where the limits of using militaries are,” said Kamradt-Scott. “More research is warranted to develop a set of universally agreed guidelines about what militaries can do in health emergencies.”

In 1999, Rocklyn Williams of the Institute for Security Studies in South Africa famously argued that in the absence of Cold War politics and conventional military threats, his country’s armed forces should focus more on “non-military” security threats. Some of these functions were identified as peace support operations, disaster relief, and medical services. “What is…required is a paradigm shift that allows for the creation of new concepts and theories capable of explaining the role and functions of armed forces in an increasingly complex, postmodern world,” Williams wrote. This was at a time when the HIV/AIDS virus was posing its own internal security threat to South Africa.

Since then, various experts have advocated for greater ties between health and the military. They cite not only global pandemics but also the increasing scale and frequency of natural disasters, as well as post-conflict reconstruction programs in Iraq and Afghanistan (which often contain a health component), as further evidence of the increasing need to reinforce links between health and security.

Despite these calls, proposals to expand the definition of a security threat to include pandemics and other risks to human security are still controversial. Understandably, they can result in fierce backlash from humanitarian agencies such as Médecins San Frontières, which are typically opposed to non-civilian engagement in these areas (ironically, MSF was one of the first organizations to call for a military response to the Ebola crisis).

Indeed, WHO guidance on civil-military coordination recommends military assets should only be used to provide healthcare as a “last resort.” While this is particularly true during conflict situations where there is high potential for blurring the lines between military forces and humanitarian actors, the report also cautions against deploying national armies and civil defense forces even during peacetime emergencies.

Yet at a time when international and domestic armed forces are playing important—even decisive—roles in fighting global pandemics, this complete aversion to military involvement may be doing a disservice to the many individuals affected by these preventable diseases. Further discussion may be needed to determine when and under what conditions the military can do more good than harm, whether that may be through providing training, transport, or security escorts, and/or by administering direct patient care, quarantine, or crowd control measures.

The time may be ripe for military, health, and humanitarian actors to come together to establish guidelines and parameters on the appropriate use of military assets during health emergencies. Doing so would ensure that the full resources at our disposal are brought to bear—as during the Ebola crisis—to save lives.