The correct answer is option 4: Virus.

Bacteria (option 1) are single-celled organisms that can cause diseases, but they are not responsible for diseases like polio and chicken pox. Protozoa (option 2) are also single-celled organisms, but they generally cause diseases like malaria or dysentery, not polio or chicken pox. Algae (option 3) are photosynthetic organisms and are not known to cause diseases in humans.

Viruses (option 4) are microscopic infectious agents that can cause diseases like polio and chicken pox. They are not considered living organisms as they cannot reproduce or carry out metabolic processes on their own. Instead, they invade host cells and use the host`s cellular machinery to replicate. This process can cause damage to the host and result in disease. Therefore, viruses are responsible for diseases like polio and chicken pox.