Ebola and Marburg viruses are single-stranded RNA filoviruses, which are zoonotic and are often transmitted through direct contact with infected fluids. Both Ebola and Marburg viruses have historically high mortality rates, which have varied from 25% to 90% and 24% to 88%, respectively ^[1][2][3,4]. The largest outbreak of Ebola virus disease (EVD) occurred in West Africa from 2014 to 2016 with more than 28,000 cases ^[3][5]. However, both viruses’ MVD case fatality rate is known to be 50%. Both viruses are transmitted through direct contact with infected blood and other bodily fluids. The Ebola virus is believed to have been first contracted by wild animals (such as fruit bats, porcupines, and non-human primates), then it spread to humans, in which human-to-human transmission is possible ^[1][3]. The first outbreak of Ebola was recorded in 1976 with an 88% mortality rate ^[3][5] and there continue to be outbreaks, mainly in Africa ^[1][3]. Marburg virus was first identified in Germany and Serbia in 1967, where laboratory workers were exposed to infected monkeys from Uganda ^[2][4].

The wide and fluctuating range of case fatality rate (CFR) data is not truly understood; possible reasons include differences in health status (nutrition, immunity, and co-infection status), genetics (ethnicity-dependent haplotypes or random polymorphisms), health-seeking behavior, case recognition, and the accessibility of supportive care at health-care facilities ^[3][5].

Zika and Dengue viruses are single-stranded RNA flaviviruses often transmitted by arthropods, such as mosquitos and ticks. Both viruses are transmitted through mosquitos. Although the Zika virus was first identified in 1947 in Uganda, it became more well known when an outbreak occurred in Brazil in 2015, leading to its spread to other parts of the Americas. Before then, it was only sporadically prevalent in Africa and Asia. Its primary concern for public health is how the virus affects pregnancy and newborn babies. A total of 5–15% of infants of women who were infected with Zika while pregnant suffered from Zika-related problems, like the following: microcephaly in the newborn, which is where the baby is born with an underdeveloped head and brain, resulting in neurological disorders and other developmental problems ^[4][6]. Dengue has become more prevalent in the past decade, from over 500,000 cases in 2000 to 5.2 million in 2019. Dengue mostly affects Southeast Asia, which accounts for 70% of the global burden. It also seriously affects the Americas and the Western Pacific region. The largest recorded dengue outbreak occurred in 2019, in which the Americas reported 3.1 million cases, and Asian countries, like Bangladesh, Malaysia, Philippines, and Vietnam, recorded many cases as well. Dengue’s severity ranges from being asymptomatic to fatal. The majority of those who contract dengue have mild to no symptoms, in which case they will get better in 1–2 weeks. Common symptoms include high fever, severe headaches, body aches, and rash. Rarely, dengue can become more severe, in which case, symptoms can also include bleeding and even cause death. One who has had dengue in the past is more likely to contract severe dengue the second time ^[5][7].

MERS-CoV, SARS-CoV, and SARS-CoV-2 are single-stranded RNA coronaviruses and transmitted via airborne particulates. Middle East Respiratory Syndrome (MERS) was first reported in Saudi Arabia in 2012, and all cases of MERS have been traced back to the Middle East. MERS-CoV is first contracted from camels, mainly in the Middle East, Africa, and South Asia, and is then transmitted to humans. However, its specific transmission route between camels and humans still needs to be determined. Due to its limited person-to-person transmission, though possible, it is not considered an epidemic; however, it still poses a global threat because of its high mortality rate ^[6][8]. Both SARS-CoV and SARS-CoV-2 originate from China, emerging in 2002 and 2019, respectively. Both viruses caused and continue to cause a global threat due to their high transmission rates ^[7][9]. COVID-19, caused by SARS-CoV-2, is still considered a pandemic as it continues to mutate and spread worldwide.

Avian influenza virus is a strain of the single-stranded RNA influenza A virus that is transmitted between birds through airborne transmission, thus being able to be detected in the form of aerosols. Avian influenza includes the strains H5N1 and H7N9, which are transmissible to humans from birds and considered the most common strains to infect people ^[8][10]. Although it cannot be transmitted from human to human, nor is it common for it to be transmitted to humans from birds, it has the potential to become a global threat due to its high fatality rate in humans. With increasing numbers of mammals with avian influenza being detected, it poses risks of the virus being able to infect humans faster and easier since mammals are biologically closer to humans than birds are. Scientists have found mammal-to-mammal airborne transmission possible in ferrets, and only a few mutations take place before it is possible ^[9][11]. Scientists have also found a strong possibility of co-infection with avian influenza and the more common human influenza due to the high seroprevalence of both viruses in minks ^[10][12]. Thus, the possibility of new mutations arising from co-infection could lead to a greater risk to public health.

Since May 2022, there have been multiple outbreaks of Mpox (previously known as monkeypox) worldwide. It has been declared a public health emergency by the World Health Organization (WHO) as it becomes increasingly transmissible among humans. The monkeypox virus is a zoonotic double-stranded DNA virus in the Orthopoxvirus genus, which also includes smallpox. Although its fatality rate is not high, it spreads rapidly, resulting in many hospitalized patients. Mpox was first discovered in Denmark in 1958. Since 2005, Mpox outbreaks have been sporadically occurring, mainly in Africa. Thus, the sudden spread of Mpox throughout Europe and the Americas caused concern. The monkeypox virus’ primary mode of transmission is through direct contact, particularly affecting men who have sexual contact with other men ^[11][13]. A summary of emerging viruses is described in Table 1.