"Write what you know" is an old saying I bet most writers have heard, if not tried to employ. For some that means researching topics you don't know so you can write intelligently about them, for others it means sticking to what they already know. It's a good mantra because it becomes painfully obvious when a writer is talking out his ass and the only thing you're certain they DO know is how to tap the keys on their keyboard like one of those hypothetical monkeys banging out the Complete Works of Shakespeare.
The Aliens have resurrected a particularly lethal strain of the Spanish Flu, with which they intend to wipe out all of humanity and make room for their own people. In a prior episode the virus escaped a makeshift lab aboard a Russian ship and killed all the humans on board.
Sophia, the leader of the Aliens, says they need a better test case than the ship. The virus was "concentrated" and in an "enclosed space" so they don't know how truly lethal (or not) the strain is. If it kills people before they can spread it then it's of no use to the Aliens. So they hatch a plan to release the virus into a shopping mall full of people and monitor how long it takes people to die.
A couple of the humans, FBI Agent Collier (who acts more like CIA - but that's another issue) and fugative Sean Walker both know about the virus and plan to stop the field test. By the time they get to the mall the Aliens are connecting the device to the building's ventilation system. The "bio-bomb" consists of a pressurized canister into which the virus was introduced, an electronic timer, and some refrigeration lines. This, of course, would NEVER work.
First of all, the Aliens are immune to the virus, so what is the point of the timer on the bomb? Why did they even bother connecting it to the ventilation system at all? They could have literally walked through the mall exposing people to the virus (which is contained in the extracted lungs of a flu victim exhumed from the Siberian tundra). Furthermore, in order to put the virus in the pressurized canister, the Alien operative opens the container with the lungs in it outside on the roof of the mall - which should have released it into the atmosphere following the show's "logic" in how this virus spreads (more on that later). So the bomb itself is unnecessary for their field test, as is a timer to set it off after the Aliens are safely out of the area - because the virus doesn't affect them they're already safe!
Ok, for a moment lets assume the bomb were necessary because for some reason the Aliens didn't want to be at the scene when it went off. Plumbing it into the refrigeration lines on the air conditioning system wouldn't disperse it into the mall! Air conditioners do NOT work by blowing refrigerant into a room, the refrigerant is a self-contained loop system. All their device would do is release the virus into a pressurized closed-loop system. Why can't we beat these Aliens if they're such morons?
Oh yeah, because the humans are pretty stupid too. Sean Walker tries to disarm the bomb - and they've gone out of their way in the show to prove he's highly intelligent and resourceful. In fact they only knew where the Aliens were because he hacked into a rental car company's real-time GPS tracking to find the Alien's rental car. Walker starts pulling wires out of the bomb and can't make head nor tail of it, he reasons there must be a power source, there must be a solenoid that will open the valve - then as the counter ticks down he panics and SHOOTS THE BOMB! There is a pressurized container filled with the deadly virus screwed into the bottom of the bomb and he thinks shooting at it is a good idea? Ok, granted he admits he panicked, but he needn't have worried in the first place. After all, it was just going to release the virus into the pressurized refrigerant lines - but even if we suspend our disbelief further for a moment and assume that somehow those metal lines were capable of releasing the virus into the ventilation system, the easy solution would have been to just CRIMP THE LINES. Ever had a car with an A/C system that has a crushed line? Yeah, it doesn't do anything because it can't circulate the refrigerant.
At the end of the episode we see that the Aliens scrubbed the field test at the mall and instead used a back-up plan - releasing the virus on a city bus. Um, but wait, didn't Sophia complain earlier that releasing the virus on the ship wasn't a good test because it was too concentrated and confined. It was an ocean-going freighter - vastly larger than a city bus. Releasing the virus on a bus would have been an even worse field test!
But that's not all, folks, she then goes on to discuss the underlying problem with the virus. It's TOO deadly. It kills humans before they have a chance to infect others. They need to make the virus a little less deadly. At that point she and her chief scientist, Dr. Lu, discuss how viruses mutate in intermediary species before they "jump" to humans in a different form. Sophia might want to get a different doctor on this because her explanation was wrong. Dr. Lu mentions "Bird Flu" becoming "H1N1" when it jumped to humans. Except H1N1 is Swine Flu, it is H5N1 that is Bird Flu. Never mind that viruses can mutate within a species too, they don't HAVE to jump species to mutate, but they do have to mutate to jump species.
At the very end of the episode they decide to infect the only "hybrid" half-human-half-alien among them to create this less deadly version of the Spanish Flu. I assume the writers thought this made sense because humans die instantly from it, the Aliens are completely immune to it, so obviously the logical way to make the virus less deadly is to put it in a human-alien hybrid who theoretically is only partially immune, right? What?!
First of all the reason the Aliens are immune is most likely because they were exposed to either that virus or a similar one and have antibodies to it. There can also be a genetic predisposition to viral vulnerability, which is obviously what the writers were also thinking - that the hybrid (Sarah) has at least partially inherited this vulnerability from her human mother. Except that doesn't really make any sense either - one theory for why the Spanish Flu has never re-emerged is that it killed off everyone who was predisposed to be susceptible to it, meaning every human alive right now is likely immune to strains of the Spanish Flu anyway.
They've already established that the virus is delivered by airborne vectors, so why did they find it necessary to INJECT the hybrid with the virus? Oh yeah, because the writers totally don't understand how viruses, particularly flu strains, are transmitted through the air. When people sneeze or cough they aerosol sputum carrying the virus, that can subsequently be inhaled by others thus infecting them as well. But the writers have assumed this virus acts more like being exposed to gas. Granted the bio-bomb conceivably could have been set up to aerosol some kind of carrier medium laden with the virus, but it would still have been more effective to set that off in the middle of the mall rather than blowing it into the air ducts.
Lastly, their experiment to inject the hybrid girl with the virus is less likely to produce a less-deadly version of it and more likely to mutate into a strain deadly to the Aliens. Why didn't they inject it into a pig or a monkey, since they'd already discussed mutation occurring in lesser species than humans. The human-alien hybrid is genetically more advanced than a regular human - she represents a "lesser species" to the Aliens, and therefore would be a perfect incubator for a mutation that could jump into the Aliens and kill them as well. It is possibly the single most stupid thing they could have done.
Yeah, I think I'm probably done with this series.