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.