Yes, it is the possibility of "catching" bacteria from each other.
To the best of my knowledge the three "bad bugs" that you want to try and avoid are:
Pseudomonas aeruginosa (we refer to it as PA - pronounced soo-dough-moan-us)
Burkholderia cepacia (also referred to as B. cepacia or just cepacia - pronounced sa-pay-sha)
Methicillin Resistant Staphylococcus aureus (MRSA -- pronounced mersa)
(and they all probably have several stains to them)
The CFF and each hospital and clinic have developed guidelines to follow to help prevent the spread of the bacteria (cross contamination) How rigorously these are followed varies from each location.
I think just about everyone has heard of the <b>three foot rule</b>.... if you are in a room with others with CF, stay at least three feet away from each other. Don't cough at or near each other, don't shake hands or hug. You want to follow the same kind of common sense rules as trying to protect yourself from getting a cold when someone else has it - wash your hands frequently, don't share drinks or utensils ... things like that.
All these bugs are in the environment all the time too so it's not like you will <b>only </b>pick them up from another person. But since doctors know that bacteria taking up residence in the lungs is enemy number 1, they try to reduce the chance of cross contamination when they can.
Some people take this very literally and follow it to a tea and others don't care about the possibility and have formed many "in person" friendships - fewer still have dated and or gotten married.
It used to be quite common for everyone to go to CF camp -- I have heard stories from our doctor and nurse about in the 1980's how wonderful it was for all the kids to get to go spend time with each other -- they shared such close living quarters that many of them died within a couple of years as a result of getting infections from one or both of the top two bacteria listed above.
There are still a few CF camps out there and they are run entirely different than they used to be. They follow much better guidelines and you have to have a clear sputum test prior to leaving for camp with a note from your doctor stating you are clear of certain bacteria (usually it's the b. cepacia that they are most concerned with)
Somewhere buried in my list of links, I have information about a camp somewhere that is free to those with CF and one person of their choice to bring with them. Everyone stays in their own hotel room and have to provide their own transportation back and forth to the group (usually outdoor) activities. This way they can get to hang out together and do group stuff but not sharing their living space or close quarters like riding in a car together - generally being outdoors and 3 feet away from each other cuts down on the chances of cross contamination.
We usually tend to err on the side of caution - having friends over the internet will just have to be good enough. I also take added precautions while at clinic visits - we wash our hand or use hand sanitizer while there and on our way out the door.
One time a girl with CF about my daughters same age at the time was doing a school project about CF and was putting together a video. She wanted to do an "interview" with anther person with CF. Both girls had only had PA one time in their lives and had had clear cultures for quite some time after that. Neither of them were culturing any bugs at the time and neither of them were even really coughing at all. All of the parents and the two children decided that there was very little risk and agreed to meet and they just followed all the 3 foot rules and all was well - I had asked the doctor about a brief outdoor meeting for the interview and about the likelihood of passing bacteria from each other -- I directly asked "generally speaking" is it more likely to pick up bacteria from another person with CF or from the environment. His answer was the environment for sure since the bacteria are everywhere! But also he couldn't "under the guidelines set forth, "recommend" the girls meet.
There is always discussion on here about where to draw the line on activities and possible exposure to bugs from not only others with CF, but other places not involving other CFers, like daycare, school, birthday parties etc. Just like every other choice, you the parent have to make, you can educated yourself and make your own decision on what works best for you rfamily and what you feel comfortable with doing.
Now..... having said all of that -- what about siblings being raised together in the same household?????
We are such an odd ball case that this didn't really apply to us much as the kids were growing up, since for so long we never even knew they both had CF. Once we knew they both had CF, we did just take extra care about not sharing a drinks or food anymore. I asked the nurse about siblings and she said that generally speaking most siblings ended up culturing the same bugs most of the time and other than recommending they not share drinks/food or toothbrush (gross!) we should just live like a family and not worry about it.