For me I bought a place infested with bugs.  

During our initial walk-thru the house, it had a HORRIBLE smell in the kitchen, an equally horrible smell in the family room (pet urine), and we found that the kitchen drawers and cabinets were infested with bugs - we had no idea what they were. 

Your end plan needs to be to get rid of this tenant permanently asap before she causes a infestation in the entire building. 

I've gotten response rates as high as 30% from sellers who have severe code violations (not just tall grass or broken windows but mold infestation, or even structural damage). b. 

You don't want someone getting sick or taking you to court because of an infestation. 

I would have called our infestation minor to maybe start getting into intermediate.

Our property had shown signs of termite damage to a structural beam during the initial inspection, and I paid for both a structural engineer (to verify the beam’s integrity was sound) and a termite inspector (to verify there was no active infestation).  

@Derek LogueI agree that if you are investing in low income it will be infested with criminals of all sorts primarily due to the fact that their lives are a mess, quality landlords (B class or better) will not rent to them and that is where they belong.

I would raise there rent to full market (they are not good enough tenants to deserve a reduced rent) and mention the filth being unacceptable and liable to attract insect/rodent infestations.

My tenant has brought to my attention that there’s a full blown flea infestation occurring in the house.