The current outbreak of Ebola virus disease (EVD) in West Africa is the largest in history, and has already killed over 1200 people. Those living in more developed areas have become fairly sheltered from the devastating effects of widespread infectious disease over the last 60 years or so, due to widespread availability of vaccines, competent healthcare, and education about hygiene’s role in disease transmission. Ebola is particularly unlikely to cause a pandemic in the developed world, as the method of transmission requires close contact with infected people.

However, that doesn’t mean that it’s completely impossible for diseases to become widespread in more developed areas. Check out these 5 diseases that could easily cause a global health emergency:

Influenza

Around 3-5 million people around the world will become severely ill due to the flu virus every year, resulting in 250,000-300,000 deaths. That’s an incredible amount of people, even though the mortality rate is fairly low. However, this is just the seasonal flu. What about more novel strains, such as those typically found in nonhuman animals? Influenza virus subtypes H5N1 and H7N9 contain strains that are found in birds that have the potential to cause disease in humans.

Some strains of H5N1 are highly virulent, and have wiped out tens of millions of birds and has had a 60% mortality rate in humans. The virus can pass from infected bird to human, but does not readily spread from person to person… yet. H5N1 mutates rapidly, and it wouldn’t take much for it to become easily transmissible.

H7N9 is particularly troubling, because we don’t know that much about it. The first confirmed case of human illness was in March 2013, with a mortality rate of roughly 22%. However, the fact that it is so new and it can be transmitted from birds that don’t even appear sick is fairly troubling, as it could become a widespread problem.

Also, it would be prudent to mention that the H1N1 pandemic of 2009 was caused by a strain that was astoundingly similar to one found in pigs, leading to the nickname “swine flu.” During that year’s flu season, the majority of cases were due to H1N1. From April 2009 through May 2010, there were over 18,000 laboratory-confirmed deaths due to H1N1. However, some estimates have put the actual number of deaths at nearly 15 times higher, citing a lack of laboratory testing in the hardest-hit areas as a reason for the underreporting. The pandemic ended in 2010, and now the strain makes the rounds seasonally.

Flu vaccines need to be administered each year because of the incredibly high mutation rate. Each passing year brings the possibility of another strain that could have devastating consequences.

Diarrheal Disease

Diarrhea is the number one cause of death in children under 5 around the globe. It affects over 1.7 billion people each year, resulting in the deaths of 801,000 children alone. Diarrhea depletes the body of necessary fluids and salts and if this is left unchecked, the infected will die of severe dehydration.

The main cause of diarrheal disease is rotavirus in children and norovirus for adults, though there are other bacterial and viral causes as well. Cholera, salmonella, Nipah virus, and hemorrhagic fever including Ebola, Lassa, and Marbug are all noteworthy sources of highly infectious diseases that cause diarrhea.

It is most easily transmitted through direct or indirect contact with feces. If an infected individual defecates near a water source, it becomes contaminated. This water could be drank directly by other individuals, exposing them to the disease, or it could be used to for crop irrigation, which can create an indirect exposure to the virus.

While most of the deaths from diarrheal disease are in developing areas that lack access to water sanitation and proper hygiene needed to avoid disease, a mutated or drug-resistant strain of the virus could severely exacerbate the disease and make things a lot worse for people in developed areas.

Tuberculosis

Tuberculosis (TB) is a bacterial infection that affects two billion people a year, resulting in 1.3 million deaths. Once exposed, an individual might not become sick right away. In fact, the bacteria can remain dormant for several years before causing illness. Though TB doesn’t spread while in this latent phase, it can become an active infection at any time.

Once symptoms begin, the lungs are the primary affected organ. What might begin as a nagging cough or fatigue can easily turn into sharp chest pain and coughing up blood. If TB affects other parts of the body, it can lead to joint pain, impede liver or kidney function, cause meningitis (swelling of the membranes that cover the brain and spinal cord), or fatal inflammation and fluid buildup around the heart.

TB is spread through the air, so being near someone with an active infection while they are coughing, sneezing, or possibly even talking is a possible means of infection. Those who are immunocompromised, have diabetes, have poor diets, smoke, or work in crowded, unsanitary conditions are more likely to acquire TB.

Unfortunately, treating TB isn’t as simple as taking antibiotics for 10 days. Many strains that cause TB are resistant to most treatments, so those with active resistant infections must take a large cocktail of medications for about a year and a half. Even still, the mortality rate for drug-resistant TB is about 80%. As drug resistance continues to grow, TB could affect even more people and result in even more deaths.

Respiratory disease

The outbreak of Severe Acute Respiratory Syndrome (SARS) in 2003 spread to more than 24 countries on four continents. SARS, caused by a coronavirus, killed 774 of the 8,098 (9.5% mortality rate) who had been infected during close contact while traveling internationally. It manifested in flu-like symptoms including fever and muscle aches, but the majority of cases developed into pneumonia or kidney failure, which can be life threatening. Due to swift and dedicated international collaboration by the affected countries, the outbreak was contained only five months after it was first reported.

Middle East Respiratory Syndrome (MERS) has reached 20 countries since it first appeared in 2012. Out of the 837 confirmed cases, 291 people have died. The virus can have an animal reservoir, which is believed to be bats and/or camels in this case. MERS is very similar to SARS in terms of symptoms and complications, though lack of healthcare infrastructure in the endemic region might explain why the mortality rate (30%) is so much higher for MERS than SARS.

MERS and SARS are both caused by highly virulent strains of coronavirus. There are six known strains that cause human illness, though the others cause fairly mild symptoms comparably. However, mutated strains or resistant to treatment could increase the number of people who become infected.

Antimicrobial Resistance

Antibiotics have been one of the best advancements in medical history, but irresponsible use has turned this blessing into a curse. When not taken correctly, the medication can’t kill all of the bacteria. The ones that are left have now been exposed to the drug, but didn’t die from it. For whatever reason, a random mutation in the bacteria’s genome has given it an advantage of resistance to that antibiotic. By not taking the full course of antibiotics, that mutation that confers resistance has been selected for, and those bacteria will continue to proliferate. When they go on to cause an illness again, they are not so easily defeated.

That has basically been the story of how things like Methicillin-resistant Staphylococcus aureus (MRSA) have become the problems that they are today. Over two million Americans develop a drug-resistant infection each year, resulting in over 23,000 deaths.

While vaccines and hygiene have eliminated much of the burden of infectious disease from developed areas, antimicrobial medications have played a vital role in curbing the spread of disease. WHO released a report in April 2014 stating that “a post-antibiotic era … far from being an apocalyptic fantasy, is instead a very real possibility for the 21st Century.”

This isn’t just a problem with bacteria; it extends to viruses, fungi, and parasites as well.

Traditional treatments are meeting resistance for a number of infections and diseases including (but most definitely not limited to) urinary tract infections, tuberculosis, malaria, HIV, influenza, Clostridium difficile (C. diff), and gonorrhea. In fact, there have been reports in several countries that gonorrhea has even become resistant all forms of treatment.

Sir Alexander Flemming discovered penicillin in 1928; which led to him being awarded the Nobel Prize in Physiology or Medicine in 1945. During his Nobel lecture, he warned against casual use of antibiotics, as they could easily form resistant bacterial strains that would have devastating consequences. Seventy years later, here we are.