Ask about the biggest problems of the world right now or recently. Terrorism, Natural disasters, Accidents etc...
Start with Natural Disasters. Ask the students to finish each sentience as much as they can and then go through it and ask as many questions as you can for each disaster. This can take a lot of time.
Move to Man Made Disasters. Explain the difference between man made and natural. Discuss and finish the sentences. Talk about how can we prevent these from happening.
Go to dangerous diseases. The lesson contains some of the most dangerous diseases but feel free to talk about many others. Key is to turn it into a discussion about symptoms/causes etc.