The HIV spreads through following types of transmission

• Sexual intercourse: it is the most important mode of transmission. Sexual intercourse through anal, vaginal, or oral route with an infected person. It can be transmitted from men to men (homosexual) men to women and women to women. The pertinent thing is that the transmission from woman to men is less than from men to woman. The viral transmission is facilitated when either partner has sexually transmitted diseases (STDs) genetical ulcers etc.
  
•  Transfusion of blood and blood products: The second higher mode of HIV transmission is through transfusion of HIV infected blood products like factor VIII
  
  
• Contaminated needles and syringes: Transmission happens through syringes, needles and such other equipments contaminated with inflected blood without cleaning and sterilization.
  
  
• Organ and tissue donations: a)Semen b)  Kidneys  c)skin  d) Cornea  e) Bone marrow.

• Mother to child: a) In uterus b) at birth c) through breast milk.

The invasion of HIV on Immune system
HIV is immuno suppressive because it infects the cells of the immune system. The major target of the prime immune cells of the body i.e. Helper T cells of T4 cells or CD4 cells. Not only the T4 cells but also the immune system come under the invasion of HIV in a phased manner.

Whenever the HIV infection takes place the immune system will come into operation in its natural way. The specific and dangerous capability of the HIV virus is that it can evade the antibodies produced against it in a mimic way.

Hence the antibodies are unable to reach and neutralize the viral effect. Simultaneously the virus infected cells are activated and starts producing the cellular products which the virus can utilize for its replication. The other important thing is HIV virus replication take place in a faster way when the immune mechanism is involved to fight down other infectious agents, thus other infection helps to the replication of the HIV virus.

One of the most striking features of HIV disease is profound depletion of CD4 Iymphocytes.  However, to the amazement of early investigators it appeared that very few of CD4 Iymphocytes could in fact be shown to be infected by the virus. In these earlier studies only 1 in 1000 or 1 in 10,000 CD4 Iymphocytes had evidence of HIV infection. Even with more modern techniques especially using the exquisitely sensitive PCR technique (polymerize chain reaction) evidence of the virus can be demonstrated in as few as 1 in 10 CD4 Iymphocytes.

Cellular Targets for HIV infection

• CD4+T-Lymphocytes.
• CD4+Monocytes and macrophages including microglia.
• CD4+dendrite cells including Langerhans cells attachment or presentation to  i) Follicular dendrite cells in Iymph nodes ii) M cells on payer’s patches iii) Galactosyl are broside positive cells in brain & gut.

During the very early phase of HIV infection before seroconversion, thevirus propagates mainly in peripheral blood mono nuclear cells (PBMC) to appreciable plasma titers with little genetic variation. Primary infection may cause temporary depletion of CD4 + PBMCS HIV infection usually elects strong cellmediated immune responses (CD8 + Cytotoxic  T cells), which shortly after the humoral responses help to clear the high virus load, but these fail to eradicate HIV infection altogether.

Natural History of HIV infection

1. Primary infection: “Seroconversion” generally the time gap between (interval) exposure and response is less than 2 months following what may be much smaller doses of virus. For example in Factor VIII concentrates and Needle sticks it might be upto 6 months.

Clinical features

• Acute Febrile Myalgic illness
• Oral and Oesophageal ulceration
• Macula – Popular rash
• Generalised Lymphadenopathy

2. Early stage (asymptomatic phase)
On recovery from above said seroconversion phase illness, the infected person steps into a silent (asymptomatic) stage. Any symptoms that occur are mild but may be debilitating.

• Generalised Lymphadenopathy
• Irregular fever
• Hyper sensitivity reactions

3. AIDS – Related complex (intermediate stage)
As the infection progresses the proportion of infected CD4 Lymphocytes increases and it becomes easier to isolate HIV from circulation.
Clinical features

• Infection with pneumococcus shigella , salmonella or Hemophilus is more common.
• Herpes Zoster is more common
• Kaposi’s sarcoma may occur at this stage or later

4. Late phase of HIV infection
This is a phase of profound immune deficiency also known as late AIDS – related complex and AIDS during this period level of CD4 + Iymphocytes falls. Oral thrush commonly develops. It is an ideal period for opportunistic infection to develop since the immunity is very low below 100/ litre.

Clinical spectrum

• Pneumonia
• Tuberculosis both pulmonary and extra pulmonary
• Candidiasis oral and oral and oesophageal
• Herpe’s simplex virus infections
• Gastro intestinal: Candidiasis Oral leuko plakia and aphthous ulceration anorexia and malabsorption diarrhea pain in right iliac due to appendicitis
• Neurological manifestations Menigitis, Encephalopathy, Acute viral encephalitis Brain Iymphoma Cerebrovascular dis eases, Myelopathy Focal encephalopathy, Peripheral neuropathies, Retinopathy Several skin lesions are common.