Case in point, my girlfriend got sick with the flu 5 days ago. She is virtually symptom-free now, but of course I got sick 2 days ago and I’m still struggling with the disease. So my question is, can I transmit it back to her or once you get it you’re off the hook? If so, for how long?
She should be off the hook unless you have a different bug than the one she had.
Why?
Assuming she has a functional immune system.
IANAD. The way your body fights off a disease like the flu is by building up antibodies in the blood to fight the specific germs causing the disease. Once you recover, your body now has the ammunition to fight that specific disease if it should attack again.
This is the principle behind shots/immunizations. They inject just enough of the disease into your body so that you don’t get sick, but your immune system goes to work to create the necessary antibodies. When you do get exposed to the disease, you can fight it off.
The problem with the flu is that it is constantly changing and evolving. The flu bug you got this year is not the same bug that will attack next year, so you need a new shot each year.
Although evidently sometimes you get a related bug - I read in The Great Influenza that a lot of the people who had gotten the flu either earlier that year or the year before, I forget, had a milder case of the killer flu. Evidently it was a similar flu, or an earlier version that evolved, or what have you.
Before your girlfried got infected, she had a wide variety of antibody producing cells floating around her system. When the virus showed up, eventually one of those antibodies latched on to it. This caused the cell producing that antibody to replicate like crazy, flooding her system with antibodies that eventually helped her fight off the infection, though apparently not before the virus found its way to you. Once the virus was cleared from her system, some of the cells that were producing antibodies against that virus were kept in long term storage. Essentially, her body now has a higher level of defense against that particular virus than it did before.