Small solid structures located at various points in the lymphatic system are known as:

What is the full form of $WHO$?

Who stated that health is a state of body and mind where there is a balance of certain humors?

Match the correct group of disease, causative agent, and symptoms.

Name of Disease	Causative agent	Symptoms
$1$. Malaria	$i$. $Entamoeba \text{ } histolytica$	$p$. Diarrhoea and flatulence
$2$. Elephantiasis	$ii$. $Plasmodium \text{ } ovale$	$q$. Enlargement of lymph vessels and nodes
$3$. Amoebiasis	$iii$. $Brugia \text{ } malayi$	$r$. Arthralgia and anaemia

$A$: $ELISA$ is based on the principle of antigen-antibody interaction.
$B$: In $ELISA$,infection by a pathogen can be detected by the presence of antigens or by detecting the antibodies synthesized against the pathogen.

Into how many main types are diseases classified?