Which of the following diseases is caused by a virus rather than by bacteria or protozoan parasites?

Introduction / Context:
In general science and health education, it is important to know which disease causing agents are bacteria, which are viruses, and which are parasites. This helps in understanding treatment approaches and prevention strategies. The question presents four well known infectious diseases and asks which one is caused by a virus, distinguishing it from those caused by bacteria or protozoa. Recognising the type of pathogen behind each disease is a common multiple choice question in competitive exams.

Given Data / Assumptions:
- Tetanus is associated with contaminated wounds, especially by soil or rusty objects.
- Dengue is a fever that spreads in many tropical and subtropical areas through mosquito bites.
- Malaria is a fever transmitted by mosquitoes, especially in warm, humid regions.
- Cholera is a severe diarrhoeal disease linked to unsafe water and poor sanitation.
- We need to identify which of these is caused by a virus rather than a bacterium or a protozoan parasite.

Concept / Approach:
Dengue is caused by the dengue virus, a flavivirus transmitted mainly by Aedes aegypti and Aedes albopictus mosquitoes. This clearly makes dengue a viral disease. Tetanus is caused by the bacterium Clostridium tetani, which produces a powerful toxin affecting the nervous system. Cholera results from infection with the bacterium Vibrio cholerae, leading to rapid loss of fluids. Malaria is caused by protozoan parasites of the genus Plasmodium (for example, Plasmodium falciparum), not by bacteria or viruses. Therefore, among the given options, only dengue is caused by a virus; the others are non viral infectious diseases.

Step-by-Step Solution:
Step 1: Recall that dengue is widely described as a viral fever transmitted by Aedes mosquitoes. Step 2: Remember that tetanus is caused by a toxin producing bacterium, Clostridium tetani. Step 3: Note that cholera is caused by the bacterium Vibrio cholerae and spreads through contaminated water and food. Step 4: Recognise that malaria is caused by Plasmodium protozoa, which are single celled parasites, not viruses. Step 5: Conclude that dengue is the only disease in the list whose causative agent is a virus.

Verification / Alternative check:
Standard biology and health textbooks list dengue under viral diseases, often alongside other mosquito borne viral infections. They mention symptoms such as high fever, joint pain, rash, and in severe cases, bleeding. Tetanus and cholera are always grouped with bacterial diseases, while malaria is classified among protozoan diseases. Public health programmes also use different prevention strategies depending on whether the causative agent is viral, bacterial, or parasitic. These consistent classifications confirm that dengue is the viral disease among the given options.

Why Other Options Are Wrong:
- Tetanus, a serious disease often linked to deep wounds, is caused by a bacterium and therefore is not a viral disease.
- Malaria, a fever transmitted by Anopheles mosquitoes, is caused by Plasmodium parasites, which are protozoa, not viruses.
- Cholera, an acute diarrhoeal disease linked to contaminated water, is caused by Vibrio cholerae bacteria, not by a virus.

Common Pitfalls:
Because several of these diseases involve mosquitoes or contaminated environments, learners may focus on the mode of transmission rather than the type of pathogen. It is important to separate the vector (such as mosquitoes) from the microbe (virus, bacterium, or parasite). A simple memory aid is that dengue is a mosquito borne viral fever, while malaria is a mosquito borne parasitic fever, and cholera and tetanus are bacterial diseases. Keeping this clear distinction in mind helps avoid confusion in similar multiple choice questions.

Final Answer:
The disease in the list that is caused by a virus is Dengue, a fever spread by Aedes mosquitoes in tropical regions.