While getting ready to hit the waves by Mark's house, Jesse stumbles upon a man collapsing outside of Mark's door, and brings him inside to the good doctor. The man dies soon after, and to Mark's fear, the man died of smallpox. This forces the two to stay quarantined inside Mark's house.
Things go from bad to worse when Steve and Amanda's investigation turns up that not only did the dead man die of smallpox, he died of a biologically weaponized strain of it he acquired while breaking into a biotech CEO's house. And this strain incubated much faster than the original, meaning there is now a ticking clock on Mark and Steve's lives.
"Murder Can Be Contagious" displays symptoms of the following tropes:
- Arms Dealer: Sebastian Dupré, Lyndstrom's intended buyer for the virus.
- Artistic License – Medicine: Mark confirms with Jesse that he has type O blood like himself before injecting himself with Jesse's blood. The show does not clarify the rh of the blood (positive or negative), which two doctors should have certainly known.
- Asshole Victim: Brandon Lyndstrom was planning to sell RZ1765, a biological weapon, to an Arms Dealer, so him being shot is a more than deserved fate.
- The Atoner: Ray Dinino was already ashamed of developing the virus, but also killed a man and got two innocent people infected. So he made a plan to leave the antigen he stole where it could be easily recovered and Mark and Jesse could be saved. His buddy Peter, however, had other plans for the virus.
- Back Stab: Ray is done in via a knife to the back.
- Bald of Evil: Well, he has long hair, just not on the top of his head, Regardless, Peter Barrow has thinning hair, worked with Lyndstrom to sell off RZ1765 to the highest bidder, was eager on the bioweapon product from the start, and was willing to kill Ray in order to get his hands on the remaining virus samples.
- Bluffing the Murderer: Once the team has realized Peter was Lyndstrom's partner in selling the virus, they claim Mark is producing the antibodies that could make more antigen. As Amanda extracts Jesse's blood, she "trips" and jabs Peter. He leaves to supposedly get to work on the antigen, but when he checks and sees that Amanda did get the needle in, he floors it over to his house, where he pulls the antigen out of a mini-fridge in his garage... where Steve is waiting for him.
- Corrupt Corporate Executive: If you're willing to sell a bioweapon you developed to solve your money woes, you might be one, Brandon Arthur Lyndstrom.
- Drives Like Crazy: As Peter races to his house to get the antigen, he nearly hits cars, scrapes the roof of his car on his garage door's underside, and doesn't even put the thing all the way in. Of course, when you're potentially dying of a biological weapon, traffic regulations seem a bit of a low priority.
- Jack Bauer Interrogation Technique: When Mark injects himself to try and make antibodies against the virus, Steve bursts into Sebastian's hotel room and points a gun right at him in order to make him sing about the virus. Sebastian makes the wise decision and does so.
- Spanner in the Works: Not that Ray really planned to kill Lyndstrom anyway, but once he shot him, he figured the virus would die off by the time anyone found the body. He did not know burglar Eddie Dempsey was working in the area and broke in a couple hours later.
