When a host is exposed to antigens. which may be in the form of living or dead microbes or other proteins. antibodies are produced in the host body, this type of immunity is called active immunity. Active immunity is slow and takes some time to have its full effect. Children who had suffered from chicken pox in their childhood. may not contract the same disease in their adulthood because they have developed antibody to chicken pox virus. The active memory has memory and initiates a heightened response during the second encounter with the disease causing agent. Te second response is powerful and is quick in acting against the disease. Thu s it provides protection against the same disease as the body already has some antibodies against it. 

After infecting the human body the HIV gains entry into the macrophages.

Events that occur in the cells:

i. The virus enters into macrophages where RNA genome of virus replicates to form viral DNA with the help of enzyme reverse transcriptase.
ii. The viral DNA gets incorporated into host cells DNAand directs the infected cell to produce virus particles.
iii. The macrophage continue to produce virus and in this way acts like a HIV factory.
iv. The HIV enters into helper T-lymphocytos (TH ), replicates and produce progeny viruses. The progeny viruses released in the blood attacks other helper T-helper lymphocytes. This is repeated heading to a progressive decrease in the number of helper
v. T_H lymphocytes in the body of infected person. During this period the person suffers from bouts of fever, diarrhoea and weight loss. 
vi. The decrease in the number of the T helper cells leads to infections as the immune system of the person becomes weak. 

Microsporum and Trichophyton are the two fungus that are responsible for ringworms. Symptoms include appearance of dry, scaly lesions on various parts of the body such as skin, nails and scalp with intense itching. The fungus are generally found in skin folds such as those in the groin or between the toes because the heat and moisture help in these organs help these fungi to grow and thrive.

a) AIDS is caused by the Human Immuno Deficiency Virus (HIV), a member of a group of viruses called retroviruses. These viruses are called retrovirus because they consist of RNA genome enclosed by the envelope which is capable forming DNA from RNA by the process of reverse transcription inside the host cell.

(b) Other than sexual contact, transmission of HIV infection in humans could also occur by

(1) Transfusion of contaminated blood and blood products

(2) From infected mother to her child through placenta.

(a) RNA interference or RNAi is a method adopted to prevent infestation of roots of tobacco plants by a nematode Meloidogyne incognita.  The method involves silencing of a specific mRNA by a complementary RNA and the formation of ds RNA. The ds RNA, cannot be translated and its expression is blocked. These RNAi genes can be incorporated into the tobacco plant through vectors. The source of this complementary RNA could be from an infection by viruses having RNA genomes or mobile genetic elements (transposons) that replicate via an RNA intermediate. Since the dsRNA cannot be transcribed thus the parasite cannot survive and the plant is protected from it.

(b) Agrobacterium vectors are used for introducing nematode specific genes into the tobacco plant. The DNA is introduced in such a way that it produces both sense and anti-sense RNA in the host cells. The two RNA’s being complementary to each other form double stranded (dsRNA) that initiates RNAi and thus, silences the specific mRNA, of the nematode. The consequence was that the parasite could not survive in a transgenic host expressing specific interfering RNA. The transgenic plant therefore got itself protected from the parasite.