Human Health and Disease

Common cold is caused by rhino viruses, and AIDS is caused by the human immunodeficiency virus (HIV), a retrovirus. Both are viral diseases.
Typhoid ($Salmonella\ typhi$) and tuberculosis ($Mycobacterium\ tuberculosis$) are bacterial diseases.
Ringworm is a fungal disease ($Microsporum$, $Trichophyton$, $Epidermophyton$).
Dysentery is caused by bacteria ($Shigella$) or by the protozoan $Entamoeba\ histolytica$.
Hence the pair of viral diseases is common cold and AIDS.

Malignant tumours are masses of proliferating neoplastic cells that grow rapidly and invade surrounding tissues; cells sloughed from them reach distant sites through blood and start new tumours. This property, called metastasis, is the most feared property of malignant tumours. So the statement is correct.
Benign tumours normally remain confined to their original location and do not spread to other parts of the body.
The pain killer given to patients after surgery is morphine (an opioid), not cannabinoids.
Heroin is a depressant and slows down body functions.
Hence the correct statement is that malignant tumours may exhibit metastasis.

When a person is already infected with a deadly microbe such as the tetanus bacterium, a quick immune response is required; there is no time to wait for the body to make its own antibodies.
In such cases preformed antibodies, or antitoxin (a preparation containing antibodies to the toxin), are injected directly.
This is called passive immunisation; it gives immediate but short-lived protection.
Weakened or dead germs are used in vaccines for active immunisation, which takes time to develop.
Hence a person likely to develop tetanus is immunised with preformed antibodies.

Allergy is the exaggerated response of the immune system to certain antigens (allergens) present in the environment.
It is due to the release of chemicals such as histamine and serotonin from the mast cells, triggered by IgE antibodies.
These chemicals cause sneezing, watery eyes, a running nose and difficulty in breathing.
Drugs like anti-histamine, adrenalin and steroids quickly reduce the symptoms of allergy.
Hence they give quick relief from allergy.

Lymphocytes are produced in the bone marrow; the primary lymphoid organs, where immature lymphocytes differentiate into antigen-sensitive lymphocytes, are the bone marrow and the thymus.
The lymphocytes that migrate to the thymus and mature there are called T-lymphocytes; 'T' stands for thymus.
The lymphocytes that mature in the bone marrow are called B-lymphocytes.
The thymus is a lobed organ located near the heart, beneath the breastbone; it is large at birth and reduces with age.
Hence the letter T refers to the thymus.