PLEASE help she is killing us!?

It started out small and unnoticeable simple colds..It was not until a few months ago did I realize that its my best friend..you see I do not have any friends because I just moved here and don't go out much but last summer I met my best friend she is my age with 3 kids around my 4yr old daughters age ( i have one child) my daughter made it to her 3rd birthday never being sick! NOT EVEN A COLD! but ever since we met this family we come home and 3 days later we are sick..We are not around any people because I am a stay at home mom and I do not know anybody in this area..This woman is kind of strange..she has her own home yet is constantly running to her moms house with her 3 kids and spending days at a time there and when she gets home she carries a illness which is ped on to me and my daughter withen 3 days..her mom has a small trailor they are VERY DIRTY! GARBAGE,BUGS,POOP AND PEE IN EVERY CORNER! I only went there once this is a 2 bedroom trailor which houses 12 people when she is there with her kids...it was a joke at first..my daughter and I would go visit 3 days after we would be sick with a cold..we would laugh and say we were allergic to her.. my husband would say I was not sure it was her who got us sick because we do go shopping and stuff...so I doented a journal of sorts in my head..visit her 3 days after head cold...visit her again 2 days later stomach flu cripples my family, visit again 4 days after chest cold...most recent visited her 3 days after stomach flu again! all this occuring in the last 3 months..my 4 yr old has lost so much weight from these illnesses..we visited her for a week without getting sick but as soon as she went to her moms trailor she brought back a new more violent illness..I love her so much and her kids and they love us soooo much but I am scared of her...we went to the emergency room and they ran all sorts of tests to see if we has a immunity disorder like aids/hiv we are fine..all tests were great! nobody else ever gets us sick..like I said other then family she is my only friend here..my 4yr old loves to play with her kids but now my daughter is so scared to go there because she fears getting ill...I mean who knows whats next the illnesses just keep getting worse! She keeps calling begging to hang out and I so badly want to but I feel knowing about this I would be a bad mom to bring my child around them..What can I say to her? does this mean the end of our friendship? I will be so lonley. any advice is helpful..I hate thinking we are allegic to them...It seems she is getting these illnesses from her mom's so I told her 3 days after she gets home I can go visit the issue is she gets bored and runs back to her mom's before the 3 days are up and the cycle keeps going. ADVICE/ANSWERS PLEASE! Thank you all in advance!