As humans spread around the globe, as human societies flourish and become more dependent on trade, and because urbanization means that people leave sparsely populated rural areas for densely populated neighborhoods, it has become much easier for infectious diseases to spread. Outbreaks are frequent, even in the modern era, though medical advances have been able to alleviate their impacts. In fact, even though the human population grew tremendously in the twentieth century, as did the population of farm animals, from which diseases could jump to humans, in the developed world and increasingly in the developing world, people are presently less likely to fall victim to infectious diseases than ever before. For instance, the advent of antibiotics, starting with penicillin in 1928, saw to it that hundreds of millions of people were rescued from death due to bacterial infections between then and now. But there is no guarantee this would continue because bacteria are becoming increasingly resistant to antibiotics, so much so that doctors and public health experts such as former Chief Medical Officer for England Sally Davies have warned of an incoming "antibiotic apocalypse." The World Health Organization warned in 2019 that the anti-vaccination movement was one of the top threats to global health because it has led to the return of almost forgotten diseases such as measles.