Who is financially responsible for costs incurred by an adjoining apartment infestation?

You would first have to solve the problem of who caused the bedbugs to be in the rental unit.

I am sure your neighbor below you would not step forward and say "Yep it is my fault, I am the reason for the bed bugs being in the rental unit"

I am equally sure the landlord is not gonna step forward with an admission of fault either.

Neither would be willing to share any cost of the purchase of you getting a new mattress as well as other costly things.

Roaches and bed bugs are a hazard of residing in a community apartment or condo living community.

While you are solving this problem you are still being bitten and have the problem of bed bugs.

There are remedies to solving bedbug problems. You might consider going to your local hardware store to purchase a few remedies that might be able to refer you to products that might assist in this problem.

It might not be necessary to obtain a new bed or mattress.

You might take any action you deem necessary to include an attempt to sure both your neighbor and landlord for any possible damages, you think you have incurred. You must have proof that these two individuals were responsible and have the judge believe your testimony as well as accept any evidence you might have. You might find this to be a difficult chore.

I hope this has been of some benefit to you, good luck.

"FIGHT ON"

You are making an unfounded assumption, and that is that the bedbugs came to your apartment from the apartment below. Chances are MUCH higher that you and/or a visitor to your apartment introduced the creatures. If a neighbor from the first infestation visited you, he/she may have been the carrier. Bed bugs much more commonly are 'hitch hikers' on humans.

In either case, you need to treat your premises for the bed bugs. You do NOT have to discard anything. Such is incorrect. Rather, you TREAT your possessions as is necessary to get rid of the creatures. Your landlord is not responsible, since HE didn't bring the bed bugs to you.