With the addition of a rechargeable shield, it follows that there's less of an emphasis on health recovery. As a result, there are no more health packs or first aid kits to collect in BioShock Infinite. You've got one health bar, and when it runs out you die (which, to be fair, still means respawning at a nearby point without losing much progress).
Instead, you'll be scavenging for food and other items to recover your health bit by bit, or purchasing health recovery items at vending machines scattered around Columbia. Likewise for "salts," the new magical currency that lets you cast vigor powers (the new plasmids).