The number of people living on the earth is constantly growing and with this growth comes an increase in the number of people living in poverty.
Poverty makes hundreds of millions of people vulnerable to infectious diseases.
Half the world's population, for example, can not get the drugs they need most. Many of these people live in cities, which are often overcrowded. While for those who are better off the city has been a healthier place to live, for the poor the city can be very unhealthy. People end up living in unhygienic conditions, without clean water and proper sanitation in overcrowded neighborhoods where microbes quickly move on from one host to the next. In other words, sometimes social conditions allow infectious diseases to take an unnecessarily heavy toll on human life.