Because I have some kind of snot monster hard at work inside my head, I'm totally going to go slim on this week's post. Forgive me my wee effort, but these short and sweet points have correlating questions at the bottom that could spark some ideas in your own work.
Since the topic is being covered, as always, in fine style by my cohorts, allow me to tweak it a bit to address how the external conflict and the public stakes go hand in hand.
First a word on external conflict: All those things that oppose the main character are the external conflict. Could be the shark, the twister or the zombies (to reference my posts of the last two Wednesdays).
1.) The shark is a pretty specific external conflict, he's in the water and he's eating townspeople. On land your hero is safe. So you have to make him go into the water to have a story. Why would he do that, knowing there is a monster out there? Public stakes could be a handy way to force him into this action. In Jaws, reward money sent some folks out fishing; Sheriff Brody went because he was a good man, but it was also his job and he was an elected official so if he wanted to continue being Sheriff he had to go out and do his job.
2.) With a tornado, you're not really safe on land, but certain areas (Hello, Oklahoma) are more prone to twisters and you may need to have your hero in that area. (And yes I'm going to resist the urge to make any but this single Sharknado reference...you are welcome.) Public stakes in Twister consisted of the hero purposely getting in the path of the tornado in order to have it lift the radio transmitters that would give data back that could increase our knowledge of the big storms and help save lives. Not only was the hero's life on the line, but the information for public safety that could be gleaned from the experiment.
3.) Zombies are different from the former examples as their numbers are typically vast and they are essentially everywhere. The threat they represent is far from singular though that happens on occasion. The bigger picture is what has already been lost --every normal daily activity now has the potential to be made dangerous because of zombies...and then "every normal daily activity" doesn't exist anymore. The people left have all lost their lives as they used to be. Public stakes here are typically survival every day. Often added in are the possibility of finding a cure to stop it from happening any more, and/or finding a surefire means to eradicate all the zombies for the sake of safety going forward.
So...ask these questions and see if there are any applicable answers for your story:
1.) What is publicly at stake for your antagonist and how can you use that to force him into the fray?
2.) What is it that your hero is in the position to learn that could make a difference to or effect many other people's lives? For good or bad? What would bad guys do to keep him quiet? Who else would then be at risk?
3.)What has already been lost to disaster (small scale/personal or large scale/global) in your story? How does that increase the external conflict for your viewpoint character(s)?