Cells infected with HIV.
1.Macrophages
2.Helper T-cells(T-lymphocytes).
Events
1.After entry,RNA genome of virus replicates to form viral DNA with help of reverse transcriptase.
2.Viral DNA gets incorporated into host cell's DNA and directs the infected cells to produce viruses.
3.The macrophages produce virus and act like a HIV factory.

(i) Measles
(ii) Polio
(iii) Tetanus
(iv) Diphtheria
(v) Tuberculosis
(vi) Pertussis.

(a) Contact inhibition is the property shown by normal cells,by the virtue of which contact with other cells inhibits their uncontrolled proliferation and growth.
(b) All normal cells have cellular oncogenes (c-onc) or proto-oncogenes;when activated under certain conditions,such genes could lead to oncogenic transformation of cells,i.e.they become cancerous.

In the primary lymphoid organs,immature lymphocytes differentiate into antigen-sensitive lymphocytes.

e.g.Bone marrow,Thymus.

In the secondary lymphoid organs,the interaction occurs between lymphocytes and antigen and proliferation occurs.

e.g.Spleen,Tonsil.

1. Active immunity is produced by injecting the microbes deliberately during immunisation.
2. Antibodies are produced by the B-cells of our body in response to the antigens;they neutralise the pathogenic agents during actual infection.
3. The vaccines also generate memory B-cells and T-cells,which recognise the same pathogen on subsequent exposure and destroy them by a massive production of antibodies.

Many microbial pathogens enter the gut of human along with food. The preventive barriers to protect the body from such pathogens are as follows:
(i) The mucus coating of the epithelium lining of the gut helps in trapping microbes entering the body.
(ii) Saliva in the mouth and hydrochloric acid in gastric juice secreted by stomach prevent microbial growth.

Factors to achieve good health:
(i) Awareness about diseases and their effects on different body functions.
(ii) Vaccination/immunisation against many infectious diseases.
(iii) Control of vectors.
(iv) Proper disposal of wastws/excreta.
(v) Periodic cleaning and disinfecting of water reservoirs,tanks,etc. to avoid water-borne diseases.
(vi) Consumption of clean food and drinking water.
(vii) Exercise and relaxation.

(a) Amoebiasis is caused by Entamoeba histolytica.
(b) Symptoms:
(i) Constipation 
(ii) Abdominal cramps and pain.
(iii) Stool with excess mucus and blood clots.
Transmission 
Drinking water and food contaminated with the faeces of infected persons.
Houseflies act as mechanical carriers and transmit the parasite from faeces to food and water.

(a) 

(b) The carcinogens are asbestos, gamma rays, tobacco smoke and arsenic.
(c) Detection and diagnosis of cancer depends upon histological features of malignant structure.
(i) Biopsy of bone marrow and abnormal count of WBCs is done in leukemia.
(ii) Pap test (cytological staining) is used for detecting cancer of cervix and other genital tract parts.
(iii) Radiography, CT scan (Computed Tomography), Magnetic Resonance Imaging (MRI scan) are useful technique to detect cancer of internal organs.

It is true that once a person gets entangled in the deadly whirlpool of alcohol or drugs, it is very difficult to get rid of this habit because, in the absence of a continuous supply of alcohol/drugs, the person starts showing certain symptoms called withdrawal signs which are listed below:
(1) Anxiety  
(2) Shakiness
(3) Nausea  
(4)Sweating.