Millionaire Mode

Which of the following could replace 'vector' as used in the passage?

Directions: Read the following passage carefully and select the most suitable option according to the corresponding number in the passage.

Communicable diseases: These diseases are caused by pathogens, which include bacteria, viruses, protozoa etc. and are readily transmitted from person to person by direct or indirect contact or through a vector. The infection may also spread through air, water, milk, food or pass across the placenta from the mother into foetus.
Non-communicable diseases: Also called non-infectious or degenerative diseases, the non-communicable diseases are not transmitted by contact with the person suffering from the disease or any other agency.
Allergy: Allergy is any condition in which a person reacts in a hypersensitive manner to any substance.
Alcoholism: Addiction to alcohol or its repeated use is called alcoholism.
When a person drinks an excess of alcoholic beverage at one time, he is likely to become intoxicated.
Drug Abuse: LSD is an example of hallucinogenic drugs, so called because it gives rise to dramatic hallucinations.