What are the various routes by which transmission of human immunodeficiency virus takes place?

(N/A) $AIDS$ (Acquired Immuno Deficiency Syndrome) is caused by the Human Immunodeficiency Virus $(HIV)$.
It has the following modes of transmission:
$1$. Unprotected sexual contact with an infected person.
$2$. Transfusion of contaminated blood or blood products.
$3$. Sharing of infected needles and syringes.
$4$. Transmission from an infected mother to her child through the placenta.