In the passage ‘nightmare’ means

In the past, famine, various types of diseases and natural disasters checked population growth in many countries. At that time, man had not learnt to till the soil sufficiently to improve and increase her yield. Man was unable to do much to conquer these diseases and natural disasters were regarded as a curse of the gods for which man had no answer. Thus famine, diseases and natural disasters remained a nightmare to mankind.
With the present development in technology and modern agriculture, one would have thought that the problem of starvation should have been solved. But starvation still stares man in the face. The increase in population now accounts for the consequent scarcity of food experienced in many parts of the world. However, in some countries, man’s advancement in technology and medicine has rapidly increased the population while improved methods of agriculture and food preservation have caused steady rise in food production. Also, man can now effectively prevent and cure most diseases in the world.
Unfortunately, the working population engaged in agriculture is so small that it cannot produce enough to satisfy the gaping mouths to be fed. In various parts of the world, man has engaged himself in various jobs in order to adjust the food production to match the growing population so his zone, but population growth continues to outstrip food production measures. Incidentally, the source of food and food preservation measures can be limited unlike population growth which may not be effectively checked. Perhaps the only effective checks to population explosion may be the drastic birth control measures. But many religious sects the world over are opposed to many to these birth control measures.