Fill in the numbered blank (iv).

Sneezes, coughs, watery eyes, running noses and sore throats, are common _____(i)_____ that mark a person _____(ii)_____ with the common cold. The common cold is a viral infection that seems _____(iii)_____. The reason is that antibiotics and ordinary medicines cannot treat a viral infection. One_______(iv)_______ reason is that the conditions, under which we say that somebody has caught a cold, can be _______(v)______ about by a range of viruses that list more than a hundred.