Which of the following is a pair of viral diseases?

The correct answer is Option (4) → AIDS, common cold.

Infectious diseases are grouped by the kind of pathogen that causes them: bacteria, viruses, protozoans, worms or fungi. For a pair of viral diseases, both diseases must be caused by viruses.

Option (1) Plague, pneumonia. — Incorrect. Plague and pneumonia are both bacterial diseases. Pneumonia is caused by bacteria such as Streptococcus pneumoniae and Haemophilus influenzae.

Option (2) Malaria, AIDS. — Incorrect. AIDS (Acquired Immuno Deficiency Syndrome) is viral, but malaria is caused by Plasmodium, a protozoan.

Option (3) Typhoid, common cold. — Incorrect. The common cold is viral, but typhoid is caused by the bacterium Salmonella typhi.

Option (4) AIDS, common cold. — Correct. AIDS (Acquired Immuno Deficiency Syndrome) is caused by HIV (Human Immunodeficiency Virus), and the common cold by rhino viruses. Both are viral diseases.

NCERT Extract:
Common Diseases in Humans – “… Dysentery, plague, diphtheria, etc., are some of the other bacterial diseases in man. Many viruses also cause diseases in human beings. Rhino viruses represent one such group of viruses which cause one of the most infectious human ailments – the common cold.”
AIDS – “… AIDS is caused by the Human Immuno deficiency Virus (HIV), a member of a group of viruses called retrovirus, which have an envelope enclosing the RNA genome (Figure 7.6).”