The percentage of the population suffering from serious mental illness is: (in $\%$)

Select the correct option for the effective treatment of a disease.

The accumulation of protein called amyloid $\beta$-peptide in the human brain causes:

Assertion $A$: The spleen is a large reservoir of lymphocytes.
Reason $R$: The spleen acts as a filter of the blood by trapping blood-borne microorganisms.
Which option is correct for Assertion $A$ and Reason $R$?

Diseases like dysentery,cholera,typhoid,etc.,are more common in overcrowded human settlements. Why?

Which type of diseases are restricted to the individual and do not spread to other individuals?