Which cells does $HIV$ primarily infect and destroy?

Which of the following cells are infected in a person suffering from $AIDS$?

In which condition does the $CD_4$ count in the blood fall below $200 \times 10^{6}$ cells/$L$?

What is the correct sequence of events that occur after $HIV$ infection in a human body?
$(I)$ Viral $DNA$ enters the host $DNA$
$(II)$ Entry into $T_H$ lymphocytes and replication
$(III)$ Viral $RNA$ is converted into viral $DNA$ by reverse transcriptase
$(IV)$ $HIV$ enters into macrophages
$(V)$ Destruction of $T_H$ lymphocytes
$(VI)$ New virus particles are produced from the host cell

What is the mechanism by which the $AIDS$ virus causes deficiency of the immune system in an infected person?

$AIDS$ is caused by which type of virus?