In the early 20th century, the Spanish flu infected an estimated 500 million people worldwide and killed tens of millions. A century later, we have vaccines, antibiotics, advanced life support and high-tech monitoring networks. And, yet, disease outbreaks — from Ebola, to Zika to measles — continue to surprise even experts.

Medical historian Mark Honigsbaum has chronicled the outbreaks and epidemics of the 20th century in his new book, "The Pandemic Century: One Hundred Years of Panic, Hysteria, and Hubris."

He says there are four lessons we should have learned, but haven’t:

1. Biomedical advances are only part of the story
“I don't think we should necessarily be reassured by the fact that we have medical technologies that weren't available in 1918, because everything that scientists have discovered about the Spanish Flu tells them this was an unusually virulent virus," said Honigsbaum. "We might find that it's just as hard to contain it now as it was then.”

2. Think big picture
“If you narrowly focus on the pathogen or the epidemiology of the disease and don't take sufficient time to understand the wider ecology — the interactions between people, microbes, and other animals in which those microbes might be harbored — you really miss a chance to understand the complexities of these epidemics,” he said.

3. Question what’s known
"Anyone can fall into the trap of thinking that once we have a certain amount of knowledge, we really understand how things work [and] we don't need to question it anymore. But the lesson of these epidemics is that, as much as we've learned and as much as we know, there were always these unknowns out there, or these partial understandings, and we only learn what we're missing when the next epidemic comes along,” he said.

4. Keep it in perspective
"It's too easy to just blame the media for hyping something that turns out not to be as fearful as people thought. But the reason why the media reports things that way, is they pick up those messages from health professionals" he said. "And often [that’s] because there are these gaps in our knowledge, and there is often a level of uncertainty. It always comes back to this idea of how much we know and how much more there is to know.”