2018 marks the 100th anniversary of the 1918 influenza pandemic that swept the globe in what is still one of the deadliest disease outbreaks in recorded history. It is estimated that about 500 million people or one-third of the world’s population became infected with this virus, which occurred during World War I; close quarters and massive troop movements helped fuel the spread of disease.

In Yemen today, after three years of fighting, the numbers are stunning: In a nation of nearly 29 million, 22 million Yemenis are in need of humanitarian assistance, according to the UN. With the economy and health care system in shambles, Yemenis make desperate decisions to find medical treatment. Some take dangerous cross-country journeys to hospitals run by humanitarian groups, like Doctors Without Borders; others spend their savings at private clinics. More than half of Yemen’s hospitals are closed or partly functioning, and sometimes administrators must choose between buying medical supplies and fuel for generators. Infectious diseases such as cholera and diphtheria are rampant, reflecting the lack of treated water and other basic government services.

Watch these recommended TED-Ed Lessons and TED Talks:

What does it mean to be a refugee?
About 60 million people around the globe have been forced to leave their homes to escape war, violence and persecution. The majority have become Internally Displaced Persons, meaning they fled their homes but are still in their own countries. Others, referred to as refugees, sought shelter outside their own country. But what does that term really mean? Benedetta Berti and Evelien Borgman explain.

War and what comes after - Clemantine Wamariya
Clemantine Wamariya was six years old when the Rwandan Civil War forced her and her sister to flee their home in Kigali, leaving their parents and everything they knew behind. In this deeply personal talk, she tells the story of how she became a refugee, living in camps in seven countries over the next six years -- and how she's tried to make sense of what came after.

Why civilians suffer more once a war is over - Margaret Bourdeaux
In a war, it turns out that violence isn't the biggest killer of civilians. What is? Illness, hunger, poverty -- because war destroys the institutions that keep society running, like utilities, banks, food systems and hospitals. Physician Margaret Bourdeaux proposes a bold approach to post-conflict recovery, setting priorities on what to fix first.

Let’s help refugees thrive, not just survive - Melissa Fleming
Today's refugee crisis is the biggest since World War II, and it's growing. When this talk was given, 50 million people had been forcefully displaced from their homes by conflict and war; now the number is 65.3 million. There were 3 million Syrian refugees in 2014; now there are 4.9 million. Inside this overwhelming crisis are the individual human stories -- of care, growth and family, in the face of lost education, lost home, lost future. Melissa Fleming of the UN's refugee agency tells the refugees' stories -- and asks us to help them rebuild their world.