Infectious-disease researchers warn that the outbreak may have been spreading silently for several weeks — or even months — before being identified by health authorities. Epidemiologists are particularly concerned about the possibility of undetected chains of transmission in rural and densely populated areas, where surveillance systems and healthcare infrastructure are often limited.

World Health Organization Director-General Tedros Adhanom Ghebreyesus stated that infections have been reported among individuals with no known epidemiological links to one another, raising concerns about widespread community transmission. He also confirmed that at least four healthcare workers from the same hospital have died after contracting the virus. Public health experts believe that the true number of infections could be significantly higher than the official figures currently reported. Authorities are still investigating how several confirmed or suspected patients were exposed to the virus, and whether additional, unidentified transmission routes or clusters may exist.

As previously reported, a national laboratory in the Democratic Republic of the Congo confirmed on 15 May that the outbreak was caused by the Bundibugyo ebolavirus, which has an estimated case fatality rate ranging between 25% and 50%, depending on the availability and quality of medical care. Like other Ebola viruses, the Bundibugyo strain spreads through direct contact with the blood, secretions, organs, or other bodily fluids of infected individuals, as well as through contaminated surfaces and medical equipment. Early symptoms often include fever, severe fatigue, muscle pain, headache, and sore throat, followed in some cases by vomiting, diarrhea, internal bleeding, and multi-organ failure.

The Huge Outbreak of 2013-2016

The 2013–2016 epidemic of Ebola virus disease (EVD) was the largest and deadliest Ebola outbreak in recorded history, centered in West Africa. The first cases were confirmed in Guinea in December 2013, but the infection quickly spread throughout neighboring Liberia and Sierra Leone. Smaller outbreaks and isolated transmission events were subsequently reported in Nigeria, Mali, Senegal, the United Kingdom and Italy.

The epidemic had devastating human and health consequences throughout the region. By the time the epidemic was officially declared over, 28,616 people had been infected and 11,310 deaths recorded.

This represented an estimated case fatality rate (CFR) of almost 40%. Besides the immediate deaths, thousands of survivors suffered long-term health problems grouped under the label “post-Ebola syndrome.” The conditions that followed the recovery were often serious and debilitating and required medical treatment and follow-up care for months or years after recovery. Reported symptoms included chronic fatigue, joint and muscle pain, vision problems, neurological problems, memory impairment, depression and persistent inflammation.

The outbreak was caused by the Zaire ebolavirus, more simply referred to as the Ebola virus, which is one of six known species in the genus Ebolavirus. Four of these species are known to cause severe hemorrhagic fever in humans and other mammals, a disease called Ebola virus disease (EVD) clinically.
Signs and symptoms of EVD include sudden onset of fever, weakness, vomiting, diarrhea, and in many cases, internal and external bleeding, multi-organ failure and shock. The virus is mainly transmitted by direct contact with infected body fluids, contaminated surfaces or infected animals, so outbreaks can be particularly dangerous in health care settings without appropriate protective measures.

The Bundibugyo Ebola

This virus is related to the Zaire ebolavirus and is one of the species capable of causing Ebola virus disease (EVD) in humans, often presenting as a severe viral hemorrhagic fever. Although less known than the Zaire strain, the Bundibugyo ebolavirus has already caused several outbreaks in Africa.

This is not the first epidemic linked to the virus. The most significant previous outbreaks occurred between 2007 and 2008 in the Bundibugyo District of Uganda — where the virus was first identified — and again in 2012 in the Province Orientale of the Democratic Republic of the Congo. During those outbreaks, the case fatality rate (CFR) ranged from approximately 25% to 51%. The latest outbreak began in the Ituri Province of the Democratic Republic of the Congo. According to preliminary investigations, the so-called “patient zero” was a healthcare worker who developed fever, vomiting, and hemorrhagic symptoms on 24 April before later dying from the infection.

One of the major concerns surrounding this outbreak is the virus’s relatively long incubation period, which ranges from 2 to 21 days. This means the virus may spread silently before symptoms are recognized. Given the number of infections reported so far, researchers believe the outbreak may have been circulating undetected for several weeks or even months. Most approved Ebola vaccines and antibody treatments were developed specifically against the Zaire ebolavirus, which has caused the majority of large outbreaks. As a result, there are currently limited treatment options specifically validated for the Bundibugyo strain.

Scientists are developing several promising candidate vaccines, including experimental multivalent vaccines designed to protect against multiple members of the Filoviridae family — including all Ebola species and the closely related Marburg virus.

When it Started?

On 18 May, an international research team released the results of a modelling study (https://www.imperial.ac.uk/mrc-global-infectious-disease-analysis/research-themes/preparedness-and-response-to-emerging-threats/report-ebola-18-05-2026/) suggesting that the true number of infections in the current outbreak could be substantially higher than official reports indicate, due to under-detection and delays in case confirmation.

On 19 May, the Director-General of the World Health Organization, Tedros Adhanom Ghebreyesus, expressed deep concern about the scale and speed of the epidemic, noting that the current numbers are particularly alarming. He compared the situation with the early phase of the 2014 Ebola outbreak in Guinea, when only 49 suspected cases and 29 suspected deaths had been initially reported before the epidemic escalated dramatically.

Health experts emphasize that the virus responsible for the current outbreak has likely been circulating for weeks or even months before detection. On 17 May, the WHO declared the event a Public Health Emergency of International Concern (PHEIC). However, officials clarified that this designation does not automatically imply a pandemic, but rather signals an urgent need for coordinated international response.

Epidemiologists are currently investigating the origin of the outbreak. Preliminary evidence suggests that transmission may have been ongoing for approximately two months. Authorities in the Democratic Republic of the Congo have identified a deceased individual on 20 April, who is being considered a potential index case. Additionally, a suspected super-spreading event on 5 May is under investigation, after which further unexplained deaths were reported and are now being reassessed as likely linked to the virus. As previously established, Ebola viruses are transmitted through direct contact with infected blood and other bodily fluids, and they typically cause a severe clinical syndrome characterized by fever, fatigue, vomiting, diarrhea, and in advanced cases, internal and external bleeding, as well as multi-organ failure involving the liver and kidneys.

As of the time of writing, suspected cases continue to rise. According to the Ministry of Health of the Democratic Republic of the Congo, the outbreak has reached approximately one thousand suspected cases with more than 200 suspected deaths

What’s Will Happen Next?

It is not unusual for Ebola outbreaks to circulate undetected for weeks or months before public-health authorities recognize the full scale of transmission.

A well-known example is the 2014–2016 West Africa epidemic, which was officially declared on 23 March 2014, although retrospective investigations indicate that the first infection likely occurred in December 2013, when the index case became ill. The current epidemic appears, in some aspects, already larger than early stages of that outbreak. However, it is still too early to determine its final trajectory. Its evolution will depend largely on how rapidly and effectively control measures — including case detection, isolation, contact tracing, and community engagement — are implemented and scaled up.

Hantaviruses are a genus of viruses that include several species capable of causing disease in humans. They are primarily carried by rodents, which serve as their natural reservoirs.

In these animals, infection is typically asymptomatic and persistent, whereas in humans hantaviruses can cause two major clinical syndromes:

• Hemorrhagic Fever with Renal Syndrome (HFRS), reported mainly in Europe and Asia;

• Hantavirus Pulmonary Syndrome (HPS), also known as Hantavirus Cardiopulmonary Syndrome (HCPS), reported primarily in the Americas.

Transmission to humans occurs mainly through inhalation of aerosolized particles contaminated with rodent urine, droppings or saliva. Infection may also occur through contaminated food, or more rarely through rodent bites and scratches.

The case fatality rate of HFRS varies depending on the viral species involved, ranging from less than 1% to approximately 15%. Some hantaviruses, such as Puumala virus, are generally associated with milder disease, whereas others, including Dobrava-Belgrade virus and Hantaan virus, can cause more severe forms. HPS is considerably more severe, with reported case fatality rates ranging from 30% to 60%.

Hantaviruses first gained scientific attention during the Korean War (1950–1953), when thousands of United Nations soldiers developed a mysterious febrile illness characterized by renal failure and hemorrhagic manifestations near the Hantan River region of Korea. In 1978, the first hantavirus was successfully isolated in South Korea and named Hantaan virus after the river where the disease had been associated. In 1982, the World Health Organization officially adopted the term “Hemorrhagic Fever with Renal Syndrome” (HFRS), and during the following years multiple additional hantavirus species were identified and classified.

And the Andes virus?

Andes virus (ANDV) is one of the hantaviruses responsible for Hantavirus Pulmonary Syndrome (HPS) in South America, particularly in Chile and Argentina. The virus is primarily carried by the long-tailed pygmy rice rat (Oligoryzomys longicaudatus), which serves as its natural reservoir. In rodents, ANDV infection is generally asymptomatic and persistent, and transmission between animals occurs through excretions, aggressive interactions and grooming behaviors. Humans can become infected mainly by inhaling aerosolized particles contaminated with rodent saliva, urine or feces. Infection may also occur through rodent bites or scratches. In humans, Andes virus infection can lead to HPS, a severe disease characterized by rapid respiratory deterioration and pulmonary edema, with reported case fatality rates of approximately 40%.

A particularly significant outbreak occurred in Chubut Province, Argentina, between November 2018 and February 2019, when a rare but well-documented episode of human-to-human transmission was observed. The outbreak resulted in 34 confirmed infections and 11 deaths. Epidemiological investigations suggested that, after the initial spillover event from rodents to humans, transmission was amplified by a small number of infected individuals attending crowded social gatherings while symptomatic or in the prodromal phase of illness. Following confirmation of the first 18 cases, public health authorities implemented strict containment measures, including the isolation of confirmed cases and self-quarantine protocols for close contacts. Investigations identified the index patient as having attended a social event for approximately 90 minutes while experiencing fever and malaise. Five individuals seated near the patient later developed symptoms compatible with Andes virus infection between 17 and 24 days after exposure.

Another patient, considered a major source of secondary transmission, was linked to at least six additional infections during the early symptomatic or prodromal phase, likely due to extensive social interactions prior to hospitalization. This patient died 16 days after symptom onset. Most remaining cases had documented close contact with at least one symptomatic individual. The route of transmission in secondary cases was believed to involve inhalation of respiratory droplets or aerosolized viral particles during prolonged close exposure.

Epidemiological analyses estimated a posterior median reproduction number (R) of approximately 1.19 across the outbreak, while the estimated R value for secondary transmission events reached approximately 2.12. Although these findings confirmed the capacity of Andes virus for person-to-person transmission, such transmission remains relatively uncommon and appears to require close and sustained contact.

Why now?

On 2 May 2026, the World Health Organization (WHO) received notification from the International Health Regulations (IHR) National Focal Point of the United Kingdom regarding a cluster of severe acute respiratory illness, including two deaths and one critically ill passenger, aboard the expedition cruise ship MV Hondius.

As of 13 May, a total of 11 cases, including three deaths, have been reported. Epidemiological investigations suggest that the event represents a convergence of biological, environmental and logistical factors that allowed a likely isolated zoonotic infection to evolve into a multinational outbreak.

Current evidence indicates that the index case was infected prior to boarding the vessel, most likely through environmental exposure in southern South America, where Andes hantavirus (ANDV) is endemic in rodent populations. Investigations in collaboration with Argentine and Chilean health authorities are ongoing. Importantly, ANDV is one of the few hantaviruses with documented human-to-human transmission, although this remains an uncommon event.

The enclosed environment of the MV Hondius likely facilitated further spread. Cruise ships represent high-risk settings due to prolonged close contact, shared indoor spaces and limited capacity for effective isolation. Preliminary genomic analyses showing near-identical viral sequences among cases support the hypothesis of onboard transmission.

The outbreak also occurred within a highly interconnected international travel network. Passengers and crew disembarked across multiple countries and ports before full epidemiological tracing could be implemented, resulting in rapid cross-border dispersion of potentially exposed individuals.

Will There be a Pandemic?

At present, there is no evidence to suggest that the 2026 Andes hantavirus outbreak associated with the MV Hondius is evolving into a pandemic scenario. According to available WHO information, transmission remains confined to a limited cluster linked to close-contact exposure aboard the vessel, with no indication of sustained community transmission beyond this setting.

Although Andes virus is notable among hantaviruses for its rare capacity for human-to-human transmission, such events require prolonged and close interpersonal contact. Unlike respiratory viruses with high transmissibility in community settings, Andes hantavirus has not demonstrated efficient sustained spread at a population level. The current epidemiological pattern does not indicate a pathogen capable of widespread global transmission.

Subscribe now

Beginnings are hard.

1. Why this exists

I am creating this Substack because science has become too long a secret to keep: it is kept behind the curtain of subscription and academic opacity, and covered up by misrepresentations, conspiracy theories, and denialism.

When knowledge is accessible only to the privileged few, it weakens itself in the process. When it is misunderstood, it creates a perilous situation.

There are no stand-alone topics of biology, virology, epidemiology, or medicine, but various lenses through which we understand the same rapidly evolving world.

The VECTOR concept originated in the conviction that complexity should not be avoided and that truthfulness should not belong exclusively to those who practice their discipline.

This is my take on making science understandable without compromising its integrity and complexity.

2. My community

This is an invitation to a common path of thought, a journey in which ideas are not taken for granted but tracked, disassembled, and reassembled in much the same way vectors disclose direction, magnitude, and structure hidden by superficiality.

Curiosity here becomes a productive power, driving ideas away from certainty and keeping them moving.

This is a community for people who find questions preferable to answers, and for whom understanding is always changing directions.

3. What to expect

This newsletter will mainly focus on biology, virology, medical sciences, scientific discoveries and biological questions.

The goal is not to oversimplify complexity but to make it affordable for everyone: to turn difficult concepts into something accessible without stripping away their scientific integrity.

So:

1. You can expect one article per week, or two per months, depending on the nature of the topic

2. All content will remain free for now

3. A future paid subscription may be considered if the community grows but accessibility will remain a core principle of my project