When Shepard isn't dealing with the reapers in Mass Effect Legendary Edition, there's plenty of evildoers, political opponents, and saboteurs to deal with. Saving the galaxy would be hard enough against one opponent, but players learn over the course of the game that racial mistrust is yet another weapon in the reaper's inventory to use against a civilization struggling to survive.
The seeds of this start off as very subtle, hiding behind the Collectors and the vorcha to do their will through a plague that incriminates humans. Saving these citizens salvages more than just a handful of lives, it actually rescues the way humanity is viewed going forward. It all starts by entering the slums to find Mordin Solus.
Give The Batarian Medi-Gel
Early on, Shepard will see a sick batarian on the right side of a hallway. He's sick and it's not something mere dialogue choices can fix, though Shepard can decide to let him die or choose to scold him.
Use the paragon interrupt when prompted. This will cause Shepard to heal the batarian automatically with some Medi-Gel (don't worry, there are plenty of refills on this map and in Mordin's clinic).
Stop The Looters
The looters in the area, being human, might not seem like they need saving, but there are actually two ways they can get killed. The first is by the owner of the home destined to return and kill the thieves. The second is at the hands of Shepard who, it turns out, isn't a fan of looters.
Using either the paragon or renegade dialogue choice will save them from either of these two grim ends and, as a bonus, they'll vow to stop looting. They will also answer some helpful questions about who Mordin is and what he's up to.
Tell The Couple To Make A Run For The Clinic
There is another duo of humans in trouble, but more from the gang activity than the plague itself. They've opted to bunker down and hide it out which is, unfortunately, not the best idea. They're not seen again and it's pretty clear they didn't make it to safety.
Again, use the paragon or renegade dialogue choice and they'll decide to run to Mordin's clinic. After clearing the way, they can be found safe and sound in the clinic later on.
Rescue Daniel And His Kidnappers
Daniel is in the most immediate danger as he's being held at gunpoint. His captors, positive that humans created the plague and that Daniel's vials of the cure are actually more doses of the poison, are about to kill him. It's possible to rescue both sides (though Mordin suggests he would have executed the kidnappers later).
While either the paragon or the renegade dialogue choices will be successful, it's very important not to shoot the batarians. Sometimes Shepard gets lucky, but in this case, the batarians are quick on the draw and will execute Daniel before Shepard gets a chance to save him.