The correct answer is option 2: virus. Measles is a highly contagious disease caused by the measles virus.

Bacteria, which are single-celled microorganisms, are not responsible for causing measles. Bacterial infections are typically treated with antibiotics, but antibiotics are ineffective against viral infections like measles.

Protozoa, another type of single-celled microorganism, are also not responsible for causing measles. Protozoa can cause various diseases such as malaria and amoebic dysentery, but they do not cause measles.

Worms, which are multicellular organisms, are not responsible for causing measles either. Worms can cause diseases like intestinal worms or filariasis, but they are not associated with measles.

Therefore, the right answer is that measles is caused by a virus. The measles virus is highly contagious and is transmitted through respiratory droplets when an infected person coughs or sneezes. It primarily affects children and can lead to symptoms such as fever, cough, runny nose, and a characteristic rash.