A person can get infected with HIV through the following routes:
Unprotected sex:
If
a person engages in sexual intercourse with an infected person
without using a condom, s/he can get infected. The sexual act
can be both vaginal and anal.
Sharing of needles:
If
a person shares the needle or syringe used by/on an infected
person, either for injecting drugs or drawing blood or for any
other purpose involving piercing, s/he can get infected. Instruments
used for piercing and tattooing also carry a small risk of infection.
Unsafe blood:
A
person can get the infection, if he/she is given transfusion
of infected blood.
Improperly sterilised hospital tools:
If
surgical devices like syringes and scalpels, or even certain
instruments, used on an infected person, are used on another
person without proper sterilization, they can transmit the infection.
Parent to Child:
An
HIV positive mother can transmit the virus to child during pregnancy
or birth. Breast milk can also act as a transmission-medium.
Theoretically oral sex without condom (on men) or barriers
like dental dam, vaginal dams or plastic wrap (on women) can
also transmit the infection.
If
a person engages in sexual intercourse with an infected person
without using a condom, s/he can get infected. The sexual act
can be both vaginal and anal.
If
a person shares the needle or syringe used by/on an infected
person, either for injecting drugs or drawing blood or for any
other purpose involving piercing, s/he can get infected. Instruments
used for piercing and tattooing also carry a small risk of infection.
A
person can get the infection, if he/she is given transfusion
of infected blood. 
If
surgical devices like syringes and scalpels, or even certain
instruments, used on an infected person, are used on another
person without proper sterilization, they can transmit the infection.
An
HIV positive mother can transmit the virus to child during pregnancy
or birth. Breast milk can also act as a transmission-medium.
