Case Study: How the Cause of Malaria Was Found

Malaria is one of the oldest and deadliest diseases. It still infects hundreds of thousands of people in Pakistan every year, especially after the monsoon in Sindh and Balochistan. The story of how its cause was found is the textbook example of the scientific method at work.

Step 1: Observations

• Patients have fever with shivering and sweating that comes back every two or three days.
• Malaria was most common in low-lying, marshy and wet areas.
• People thought it was caused by bad air from marshes. The name comes from Italian mala aria, "bad air".

Step 2: Question

What causes malaria?

Step 3: The cause is found in the blood

In 1880, the French army doctor Charles Laveran, working in Algeria, looked at the blood of malaria patients under a microscope. He saw tiny living organisms inside their red blood cells. This parasite was later named *Plasmodium*.

Hypothesis 1: malaria is caused by Plasmodium. Doctors deduced that if this were true, Plasmodium would be found in the blood of every malaria patient, and it was.

Step 4: How does Plasmodium get into the blood?

Many observations linked malaria with mosquitoes: both were common in wet, marshy places, and people who slept under nets got malaria less often. In 1883, the American doctor A. F. A. King listed these observations and suggested that mosquitoes spread malaria.

Hypothesis 2: Plasmodium is carried from person to person by mosquitoes.

Deduction: If mosquitoes carry Plasmodium, then the parasite should be found inside mosquitoes that have bitten malaria patients.

Step 5: Ronald Ross's experiments

Sir Ronald Ross, a British army doctor working in India, tested this hypothesis.

1. In 1897, in Secunderabad, he let mosquitoes bite a malaria patient. Days later he cut them open and found Plasmodium growing in the stomach wall of the Anopheles mosquito.
2. Human volunteers were scarce, so in 1898, in Calcutta, he used sparrows, which get a kind of malaria. Culex mosquitoes were allowed to bite infected sparrows. The parasite grew in the mosquitoes and moved to their salivary glands.
3. These mosquitoes then bit healthy sparrows, and the healthy sparrows got malaria. Healthy sparrows that were not bitten by infected mosquitoes (the control) stayed healthy.

Conclusion: the hypothesis was accepted. Malaria is caused by Plasmodium and is spread by the bite of a mosquito. Soon after, the Italian scientist Giovanni Battista Grassi and his team showed that human malaria is spread by the female *Anopheles* mosquito.