Throughout most of human history, the population remained small, so that births and deaths were roughly equal. Many died due to disease, famine and war. It is estimated that man in the Stone Age had an average life expectancy of only 17 years, while ancient Romans and Egyptians attained 30 years on the average. (In contrast, the average life expectancy in many countries of the world today is over 70 years.) Many died as a result of large-scale epidemics which swept across continents from time to time. For example, in 14th century Europe, it is estimated that a quarter of the population died from bubonic plague. Other killer diseases were cholera, yellow fever, typhus, malaria and smallpox. Wars are also responsible for killing large numbers of people, including civilians. For example, the Second World War is estimated have cost nearly 100 million lives, and the recent Afghan war about a million lives. Wars create famine and cause disease. Despite the above checks on the growth of human population, remarkable changes were being brought about by man himself, particularly in the past few centuries, which favored the growth of human population. From the time when man began to fashion tools, to use energy other than his own muscle power, to make machines, to grow crops and protect them, his productivity has increased enormously. He has been able to increase his food and shelter resources beyond imagination, and to subdue all other living organisms of this earth. Advances in sanitation and modern medicine, particularly from the 17th century onward have practically wiped out the major killer diseases – at least in the more developed countries. One has only to think of the role of immunizations and antibiotics like penicillin in preserving life. As a result, life expectancy in most countries has risen remarkably, and death rates have dropped sharply. These advances in modern times have more than outweighed the natural checks and are to a great extent responsible for the population explosion, especially in the less developed countries.