5 Most Deadly Infectious Diseases

You may think that in the 21st century the infectious diseases are no longer a major threat to the human life. But this is far from the truth. Read the entire article to meet the killers.

5) Malaria

This one is not caused by a virus or bacterium. The organisms causing it are eukaryotic protists of the genus Plasmodium. But they don’t infect exclusively humans – but also rodents, reptiles, birds and monkeys. Malaria is widespread in tropical and subtropical regions. That’s because warm and wet conditions, where stagnant water abounds, are ideal for mosquitoes. Mosquitoes are a vector for malaria, which means they transmit malaria to us. How common and how deadly malaria really is? There are approximately 225 million cases of malaria every year, resulting in about 780,000 deaths – killing mostly children in sub-Saharan Africa.

This is a Plasmodium sporozoite. Its gift to humanity is malaria.
This is a Plasmodium sporozoite. Its gift to humanity is malaria.

4) Tuberculosis

Tuberculosis (or TB) spreads through the air in saliva (coughing, sneezing, spitting into people’s food). In most cases it results in an asymptomatic infection. However, in 10% of cases it progresses to active disease, which, if left untreated, kills 50% of people infected. Contrary to what you may believe tuberculosis doesn’t only affect lungs. Skin, eyes and other parts of your body can also be attacked. Treatment is already difficult and becomes more so with the rise of multi-drug-resistant TB cases. One in three people in the world is infected with TB (5-10% in U.S.) and a new person joins this club every second. Tuberculosis kills about 1.6 million people every year.

The world famous - Mycobacterium tuberculosis.
The world famous – Mycobacterium tuberculosis.

3) Diarrheal diseases

Also known as „stomach flu”, or – if you want it to sound more scientifically- gastroenteritis. Those diseases are caused by many different species of bacteria, viruses and protozoa. The most serious threat posed by diarrhea is dehydration. Water and important electrolytes are lost by passing liquid stools, vomiting, urinating and excessive sweating. Unless they are replaced, dehydration occurs. It doesn’t sound so difficult, does it? Just drink some nice, clean water, add some salt, sugar and have a zinc tablet. And that’s exactly the problem – for many people in poor countries none of those are easily available, resulting in diarrheal diseases leading to severe dehydration and that in turn leading to death. Small children are at most risk – but adults die too. Diarrheal diseases kill 1.5 million children alone – every year. 80% of them are under 2 years old. Yes, you read it right. So hit Thewaterproject.org and donate something, if you care. If not, just keep reading for number 2 killer disease.

Say Hi! to Salmonella typhimurium seen here in a stylish pink.
Say Hi! to Salmonella typhimurium seen here in a stylish pink.


HIV doesn’t kill directly. It causes AIDS – Acquired immune deficiency syndrome. By destroying human immune system, it allows bacteria, viruses and cancers to frolic freely inside your body. What would be a simple cold for any healthy person could be deadly for someone with AIDS. Keep reading for some scary numbers. AIDS kills around 2 million people every year. There are around 33 million infected people in the world and the number of new HIV infections every year stands at around 2.5 million.

HIV-1 virions (green) on the surface of a lymphocyte (red).
HIV-1 virions (green) on the surface of a lymphocyte (red).

1) Lower Respiratory Tract Infections (or LRIs)

Among all infectious diseases, this is the leading cause of death. Surprising, isn’t it? You’d expect some evil-sounding name of a virus, known to dissolve your organs and turn you into a living pulp. But that’s not the case. It is very likely that you are carrying some LRI-causing bacteria at the very moment in your throat and nose, you just don’t get sick. But, if you were very young or very old, and your immune system was weaker -who knows.  LRIs are very common and definitely not limited to dark, tropical forests. There are many infections capable of affecting the lower respiratory tract, but two of the most common LRIs are bronchitis and pneumonia.

In 2002, LRIs killed about 3.9 million people worldwide.

Streptococcus pneumoniae in spinal fluid. FA stain (digitally colorized).
Streptococcus pneumoniae in spinal fluid. FA stain (digitally colorized).

Source for all the images: Centers for Disease Control and Prevention

Further reading:

World Health organization Factsheet on Tuberculosis

National Health Service page on Tuberculosis
HIV InSite – a project of the UCSF Center for HIV Information
Roll Back Malaria – The Global Partnership for a Malaria-free World
Infectious Diseases Articles on Medscape