Mad cow disease in cattle is caused by an organism which has

Match the Column-$I$ and Column-$II$:

Column-$I$	Column-$II$
$(i)$ Viroid	$(a)$ Ramachandran
$(ii)$ Cell	$(b)$ Leeuwenhoek
$(iii)$ Virus	$(c)$ $T.O. Diener$
$(iv)$ Triple helical structure of collagen	$(d)$ Ivanowsky

Dahlia mosaic virus has which of the following as its genetic material?

$T. O.$ Diener discovered a new infectious agent that was smaller than viruses and has the following characteristics:
$(i)$ It causes potato spindle tuber disease.
$(ii)$ It has free $RNA$.
$(iii)$ The molecular weight of $RNA$ is low.
Identify the infectious agent.

Sometimes when a virus attacks a bacterium,neither the virus multiplies nor the bacterium dies. This phenomenon is called as

What is mainly responsible for Alzheimer's disease in humans?