Infestation - Rat/roach?

[Majour Bey]

Hey Guys, I recently took over a rental property that my father was going to allow to foreclose. The property has been rented out by primarily, the same tenants throughout his tenure. Here is my issue; this is a two family home. My tenants on one side, have informed me that they can here mice in the walls & are accumulating roaches - via the tenants on the other side. I need to take action immediately, to avoid more serious problems. However, being that I did not cover mice/roaches/infestation in my lease (I did not take this into account at all), who is fiscally responsible for the removal of all infestation - the landlord of the renter? I've just renovated a bathroom, kitchen and installed carpeting on one side. I am not against investing in my new investment. However, I do not live there - in conditions that would breed unwanted company. I am seeking insight from anyone who has dealt with this problem/ can relate and assist me in creating a swift course of action! Thank you all for your help and time.