The Deadliest Infectious Diseases

As far as I am aware there is no such thing as a good diseases but believe me some are worse than others. Whilst it is a ecological necessity that infectious disease pathogens evolve to spread more efficiently there is no obvious reason why some of them wreak such horrific deaths on their hosts. You would think it in a pathogen’s interest to keep its host alive for as long as possible to spread to as many more people as possible. But no. It’s almost as if some sadistic fiend sat down and designed these diseases to instill as much terror on the population as possible.

The list of diseases below are all infectious, all frequently fatal and all very, very unpleasant. The order reflects a mix of how deadly they are and how horrific the symptoms are.

Warning: there are graphic images lower down this page.

10. Influenza (Flu)

Spanish Flu

To most people flu is a pretty unpleasant illness contracted 2-3 times in a lifetime. However, influenza was responsible for one of the most destructive pandemics of all time – the Spanish Flu (1918-1919) which killed up to 100 million people. The particular type of flu strain (H1N1) was unusual in that most fatalities where amongst the young and healthy. It seems that the flu actually turned the healthy body’s immune system on itself creating a “cytokine storm” which attacked the lungs.

The most recent flu to hit the headlines was Swine influenza. The alarms bells started ringing when it was found to be a new variant of the H1N1 strain. Deaths did occur and it did seem particularly virulent, but nothing compared to the scale of Spanish flu. I actually had Swine flu and it wasn’t nice.

Perhaps the biggest reason to fear influenza is its ability to combine and mutate to form new strains. Chief amongst these fears is that a super-virulent strain will combine with a highly transmissible strain. This could even happen across species as is the case with the potentially lethal H5N1 Bird flu. Currently bird flu is not able to spread from person-to-person, however, just a small genetic event could open the way for a major epidemic.


HIV / AIDS virus

Human immunodeficiency virus infection / acquired immunodeficiency syndrome (HIV/AIDS) works by effectively destroying the body’s defences to any number of other diseases and infections. There is still no cure or vaccine and until very recently, no effective treatment. To date more than 30 million people have died of AIDS with nearly 40 million currently infected.

HIV increases the chances of picking up infections such as TB, toxoplasmosis and hepatitis. It also raises the possibility of developing several forms of cancer. New treatments with antiviral drugs have increased life expectancy greatly for those with HIV. However, it is the ever changing nature of the HIV virus that makes it so difficult to combat. The virus actually inserts itself into the DNA of the body’s cells, becoming part of your genetic template. Not only that but it evolves rapidly within the body to the extent that evolutionary trees can be made from virus samples taken in different parts of the body.

8. TB (Tuberculosis)

Tuberculosis - Pott's disesase
Tuberculosis can affect any part of the body

TB is highly contagious and easily spread through airborne droplets e.g. a sneeze. In fact around a third of the world’s population is actually infected with TB. Fortunately this is in the latent form and is more than likely they will never be aware. Only 1 in 10 cases progress to the active disease which has a 50% mortality rate if not treated. Tuberculosis kills around 1.5 million people worldwide every year, second only to malaria.

The disease primarily affects lungs but can affect any part of body including the testicles (ouch!). The most common initial symptoms are coughing up blood followed by rapid weight loss – this is where the name consumption came from. As the disease progresses large areas of the lungs are necrotized and turned into a cheese like substance. In extreme cases the infection can erode into the pulmonary artery of the lung risking the patient drowning in their own blood.

TB affecting the rest of the body (extrapulmonary tuberculosis) occurs in around 20% of cases. It can affect the nervous system, urogenital system and bones, as can be seen in the photo above where the spine has been infected and collapsed in on itself. In some cases TB can be invade many organs (Miliary TB) affecting the liver, spleen, brain as well as the lungs – this is a particularly serious condition. Ironically most of the tissue damage in TB is caused by body’s own immune system bombarding the infected area with cocktail of chemicals.

Whilst many cases of TB are now treatable with antibiotics new resistant forms are emerging raising the spectre of consumption once again sweeping the world.

7. Anthrax


Anthrax is a bacterial infection which in most forms is lethal. There are three ways in which a person can become infected and this is very important to how the disease progresses. The most lethal way is by inhalation. If not treated this is invariably fatal. With prompt antibiotic treatment still only around half survive.
Anthrax can be ingested, often through infected meat. The initial symptoms of this form include vomiting blood and extreme diarrhea. Outcomes are slightly better if treated with a 25-60% mortality rate.
The bacteria can also spread through broken skin. This causes a dark ulcer and may result in whole limb becoming very inflamed. However, death is rare as the pathogen cannot penetrate beyond skin.

The active ingredient of Anthrax is the imaginatively named “Lethal toxin“, although it is not actually lethal until in combination with edema factor, and protective antigen. Together theses cause wide scale tissue destruction and bleeding with dark, non-clotting blood oozing from bodily orifices. Death occurs within a few days to 2 weeks.

Anthrax is rare but has not been consigned to the history books. In recent years two  cases involving drum skins in US and UK have been reported. The mode of infection here was through the drum skins made from animals being contaminated with anthrax spores. This is one of the scariest things about anthrax it can exist as dormant spores which may survive for 100s of years. An island in Scotland was rendered uninhabitable for 50 years after anthrax experiments by the British government. It had to be decontaminated before anyone could return. It is very difficult to eradicate anthrax spores with burning being one of the few options.

Obviously something this deadly attracted the military and anthrax was weaponized by both US and USSR. In Russia 68 civilians were killed  in the 1979 Sverdlovsk incident when weapons grade anthrax accidentally leaked. Bioterrorism is also a possibility with a series of fatal postal attacks in carried out in the USA in 2001

6. Cholera


Cholera is one of the most infectious diseases and is easily passed through contaminated food and water. Historically it has ripped through communities leaving half dead, even now it is estimated to kill 120,000 per year. In the past epidemics killed millions.

Cholera kills through rapid dehydration. The symptoms are non-stop vomiting and diarrhea until you can’t anymore – and then some more. One estimate suggest patients can produce up to 20 litres (5 US gal) of diarrhea a day. What is tragic about the disease is it is very cheap and easy to treat with fluids and electrolytes. When treated only 1% of cases result in death; untreated there is an over 60% chance of fatality. The most virulent strains of cholera can kill within 2 hrs if the patient is left untreated.


MRSA - necrotizing fasciitis

MRSA, or Methicillin-resistant Staphylococcus aureus to give it its full name, is particularly scary as it renders modern medicine ineffective. The so called “Superbug” has the ability to plunge us straight back to the Dark Ages where peasants were afflicted with all manner of boils and hideous skin diseases! But MRSA is more than just a skin infection it has been known to kill within 24hrs.

There are a number of  strains of MRSA with some much more virulent than others(ST1:USA400 and ST8:USA300). But they all have one thing in common; they are resistant to a wide range of the most commonly used antibiotics that are available to mankind. In theory they should produce the same symptoms as a standard staph infection, and most of the time they do. However, not only are they not easily treatable, but in many cases appear much more pathogenic than their non-resistant relatives.

Conditions associated associated with MRSA include:

Necrotizing fasciitis  – a flesh eating condition affecting deeper layers of  the skin.
Toxic shock syndrome  – a systemic infection that can be fatal.
Necrotizing pneumonia – flesh eating infection of the lungs.
Osteomyelitis – a painful bone infection.
Sepsis – a potentially fatal infection of the bloodstream
Endocarditis – infection of the heart

MRSA can effectively eat your body with very few available antibiotics effective – and resistance growing. It seems that the antibiotic era is drawing to an end and much of this is down to people misusing and overusing them.

4. Rabies


Rabies is invariably fatal if not treated immediately after bite. If intervention only occurs after the symptoms start showing there is an 8% chance of survival. Worldwide rabies kills around 55,000 people, mostly in Africa and India, but it does still exist in the US and Europe.

Rabies first infects the central nervous system and ultimately causes disease in the brain leading to death. Symptoms include acute pain, violent movements, uncontrolled excitement/mania and hydrophobia. This is seen as panic when the patient is given liquids to drink even when thirsty and triggers excruciatingly painful spasms of the muscles in the throat. Perhaps the best known symptom though is the foaming at the mouth caused by the excessive production of saliva.

The incubation period for rabies varies greatly from a few days to several years. Unfortunately once symptoms begin death occurs within 2-10 days.
Rabies can be carried by any warm blooded animal,but in the vast majority of cases it is a bite from a dog that is the source.

3. Smallpox


Smallpox is possibly the most horrific looking of any disease. The classic symptom is the body being covered in pox, fluid filled pustules. This isn’t limited to the skin either they can occur in the mouth and throat. The disease has been around since 10,000 BC and was responsible for 300 million deaths since 1800 alone.

Whilst the mortality rate is only around one third most survivors suffer some scarring with other complications including blindness. The number of deaths is high though because smallpox is incredibly contagious.
The mortality rate depends on the course the disease takes. Of the four forms malignant and hemorrhagic smallpox are usually fatal. In hemorrhagic smallpox, most serious form, there is no blistering of the skin, instead there is bleeding under skin causing it to turn black. Given the name “black pox” this form would  kill in around 6 days.

The good news about smallpox is it has been officially eradicated. The world has been free of smallpox since 1976 with the last recorded case two-year old Rahima Banu in Bangladesh.

Of course the deadly potential of smallpox was recognised by the military. It was first used as a bioweapon by British as early as 1789 against Australian aborigines. Weaponized by Soviets during the Cold War it now only exists in laboratories. One worry is smallpox could potentially be re-engineered from digital viral genome and inserted into related pox virus.

2. Bubonic Plague

Bubonic Plague

If any disease can evoke pure terror then it is Bubonic Plague. Responsible for the Black Death which swept Europe in the middle ages killing an estimated 100 million people. There have been other plague epidemics including one in the 6th century which killed 50 million throughout the Roman Empire.

The plague is spread by a bacteria carried by rat fleas, unfortunately for them it kills them too. Symptoms occur within 2-5 days of a bite;  initially the lymph glands nearest the flea bite swell up like golf balls (buboes). Further symptoms include cramps, seizures and even vomiting blood. Acral gangrene at the extremities causing fingers, toes and noses to turn black are common and the skin may become very discoloured. This is where the name “Black Death” came from. In the latter stages there may be extreme pain caused by skin decaying on the living person.

Bubonic Plague (Yersinia pestis) still exists and sporadic cases occur even in the USA. There have been major outbreaks as recent as 1946 but nothing on the scale of the great plagues in history. The (relatively) good news is it can be effectively treated if antibiotics are given within 24hrs. Otherwise the mortality rate is in excess of 60% .

Naturally the military would not miss out on a chance to utilise something as terrifying as the plague. It is in fact one of the first ever biological weapons having been used in the 14th century. In this case diseased corpses were catapulted over the walls of besieged cities. The Japanese also used weaponized bubonic plague against the Chinese in World War II.

1. Ebola


Ebola is highly contagious with evidence that it can be spread via air. There is no treatment. There is no vaccine. Agonising death has been known to occur in excess of 90% of cases in some outbreaks. For these reasons the Ebola virus trumps any current disease to become the deadliest infectious disease on the planet.

Ebola is actually a group of viruses all of which are native to central Africa. The first reported cases in the mid-1970s appear to have been related to the local taste for bush meat, i.e. indigenous wildlife. One of the scary thing about Ebola is nobody is certain where it came from. And nobody knows for sure where it exists. It is assumed there is a natural reservoir of the virus within some animal population, but the species is unknown.

Infection with Ebola starts out with pain practically everywhere;  joints, muscles, abdominal cramps, headaches. Various rashes usually also appear. Ebola interferes with the blood’s clotting mechanism and bleeding may occur from every orifice.  Near inevitable death is usually caused by multiple organ failure and internal tissue necrosis.
Surprisingly, of those who do survive many make a complete recovery. It is worth noting though that the Ebola virus can be transmitted via semen up to 12 weeks after an infection.

In the post Cold War era no government has taken it upon themselves to try and turn Ebola into a weapon of war. But with its great potential to kill horribly it has been judged a category A bioterrorism agent.

26 thoughts on “The Deadliest Infectious Diseases”

  1. I saw a TV show a few years (on the public TV channel) about TB. It said that TB has probably killed more people than any other infectious disease during human history. It said when it was discovered how it was spread, men started shaving their beards off, and women were wearing shorter skirts that didn’t drag the ground. (Men used to spit a lot.) around the beginning of the 20th century.

      • It may not be the deadliest but they all look like terrible diseases! I gave up meat after reading this. Maintain your hygiene and wash your hands properly. Use a scrub if you want and don’t touc your eyes, nose, mouth and ears. WASH YOUR HANDS 100x TIMES BEFORE EATING, ICLUDING THE PLATE AND SPOON IF USING ONE

  2. Living in a first world country, the threat of Ebola seems minor considering its origin is mainly the African continent. I’ve had a phobia of Ebola and similar viruses that cause gruesome hemorrhagic deaths since I was six. Ebola spread from human to human, even though it is a fluid based pathogen it has the ability to mutate, like any virus, therefore the threat of potential spreading by air (which btw most airborne viruses are spread through microscopic fluid molecules we breath out). There are even questions to if the Bubonic Plague was not actually Yersinia pestis but a strain of Filoviridae because of the rate of contamination and symtoms associated wth liquification of internal organs.

    If anyone really insists on knowing how I know such things. I have been an Infectious Disease Specialist for over a decade now 🙂

      • You still believe WHO?? even after their biased behaviour towards China. he also said that china handled COVID-19 well……talk about believing china I am not against the people of china I am only against the government. WHO was pressurised by the Chinese government to come up with such conclusions.

  3. If you’re going to include Plague, which is spread by fleas and kills few people today, then why not include malaria? It’s perhaps the deadliest disease of all.

  4. Alexis is correct; Ebola is a liquid based virus and can no more become air born than zebras can fly (not my quote). At a recent lecture on health care and hospital readiness for viral threats, I’ll quote the lead doctor when I say “when making a list of the top ten infectious diseases to worry about, Ebola wouldn’t make the first 4 pages”. [The flu with over 100million deaths didn’t make the top 9 and continues to kill thousands every year??] Causes me to move on to other sites with more robust research.

    • Firstly, this site makes no claims to be the authoritative scientific literature on the subject of infectious diseases and I would assume even the most cursory glance would have established this.

      However, if you read the introduction you would have read:
      “The order reflects a mix of how deadly they are and how horrific the symptoms are.”
      And yes, this is subjective, but with a 70-90% mortality rate that ticks a few boxes.

      So, flu gets onto the list but as this is not a medical text, along with the fact the virulence of flu is very much strain dependent, it does not get into the “top 9”. BTW is the top 9 a recognised measure of import?!

      As for ebola not being airborne; yes, you are correct it is fluid based. However, I guess you didn’t look at the paper I mentioned ( ). It suggests that aerosol infection was worth exploring as a route of infection, i.e. evidence of spreading via air…

      • If there’s no measurable criteria for your list and it’s just a subjective jumble of what someone ‘thinks’ should be in the top 10, then that should be stated.
        However, the whole page loses credibility with stating that Ebola could turn air born as a delivery mechanism. This smacks of fear mongering and artificially elevating the possible threat of an Ebola infection. If the platform you’re standing on is proposing that Ebola could be spread by air, then that’s the same color we’ll use when reading the rest of your information. Incidentally, whatever page that link went to is no longer active.

        • This article was written long before Ebola became headline news – therefore it was not considered particularly sensationalist or under the scrutiny it now appears to be. However, I stand by the fact that everything written above is backed up by at least some empirical evidence, including the assertion “Ebola is highly contagious with evidence that it can be spread via air”

          If you want a list of infectious diseases ranked by morbidity, case fatality rate, infection rate etc then you are looking in the wrong place. This article is not a review of all scientific literature.

          Also, FYI link above is fixed, and more information here:

    • it can be spread by:

      directly touching the body of someone who has symptoms, or recently died from the disease
      cleaning up body fluids (blood, poo, urine or vomit) or touching the soiled clothing of an infected person – the virus can survive for several days outside the body
      touching unsterilised needles or medical equipment used in the care of the infected person
      having sex with an infected person without using a condom – studies show traces of Ebola may remain in semen many months after recovery

      • Ebola is not an airborne virus. It can only be spread by “air” through a manmade aerosol by hosing down infectious waste. People who say that it has become airborne are only fear mongering.

        • This article was written long before ebola became headline news and was not intended to be particularly sensationalist. However, I do research my topics and have studied infectious diseases. I may ammend the text slightly but there is evidence of airborne infection, e.g:

          Jaax N, Jahrling P, Geisbert T, et al. Transmission of Ebola virus (Zaire strain) to uninfected control monkeys in a biocontainment laboratory. The Lancet. Dec 23-30 1995;346(8991-8992):1669-1671.

        • And why exactly would anyone trust or take the word of the WHO on anything of any importance sine their statement in January 2020 stating that human to human transmission of Covid 19 was not possible or evident. When obviously that was completely wrong, laughably so in fact.


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