In real life, plasmids are circular strands of DNA that can be transferred between bacteria hosts. They function independently of the host's own DNA, and so can provide benefits separate from the host's normal genome. They can code for a wide variety of things, including antibiotic resistance, toxin production, and even enzymes that break down harmful chemicals in the environment.
In Syndemic, plasmids are the key to survival and success. Found on destroyed enemies or hidden within tissue cavities, they provide randomly-generated bonuses that will aid you in your fight against disease. You can have several different plasmids active at one time, but you can store dozens of inactive ones in your inventory. Their names give a general idea of what they do.
Some random examples of the plasmids you'll encounter:
regA/hisC-HERCULES: Improves your cellular health pool, allowing you to withstand additional damage from enemy pathogens.
LacB::recK-IMMORTAL: Gives your cell the ability to regenerate and repair damage more effectively.
hexT/talQ::neoG-ANGRYMAN: All antiseptic attacks are enhanced and do additional damage to pathogens.
thyP/YFP-SCROOGE: Your cell is more efficient at collecting biomass from the environment (or extracting it from enemies!).
tetA::argC-FRISKY: Streamlines your cell shape to provide an increase in manoeuvrability and maximum speed.
(Realism Note: Actual plasmids have much less interesting names than these, usually based on where they were first isolated and what sort of antibiotic resistance they have. My names are a bastardization of gene terminology and my own descriptions. But, if you want some really crazy gene names, look up the genes for fruit flies...
Also, plasmids are exchanged between bacteria via a process called Horizontal Gene Transfer. They aren't free-floating objects to be picked up and collected by any passing microbe, as in Syndemic!)