What do you do if you are renting?

I would document the problem and break your lease and look for another place.

Really nice question

Well you can't necessarily blame your tenants for this, I'll tell you that much. You can pick up bedbugs almost ANYWHERE. Take a trip and stay at a hotel--have guests who sleep over--even visit some concert, gathering, restaurant or other public place--and you can bring bedbugs back to your place without ever realizing it until you have an infestation. They are EXTREMELY easy to transmit and get. Bedbugs hitchhike in your clothing, shoes and even in library books you may get--and they stay dormant for sometimes years before starting to breed!  So do NOT automatically assume that the tenants are responsible here, or that they are not clean people.

What you DO need to do is get a professional exterminator out there, have the place treated (it usually only takes ONCE), make your tenants pay at least half of that cost, and also MONITOR them to make sure they are following the exact instructions from the pro who comes out. They will need to remove all clothing, paper, books, mattresses, blankets, towels, curtains, shoes and rugs, move their furniture away from the walls, have everything sprayed, wash everything textile in hot water and dry it in the dryer on high heat, put hard things or books into black plastic bags they can completely seal up and put them in the sun for a week, cover their mattresses with bug-proof covers--and other things the exterminator will tell them to do. You HAVE to make sure they follow through, or the infestation will take more time and effort to eradicate. 

The good news is they CAN be eradicated. Just do what the professional tells you to do. And don't cut corners doing it, or try to do it yourself. Seriously. I've been there--and it's unpleasant, but if you do what you're told, it works. 

If no one else has bedbugs, then they brought them in, and in most places, they pay to get them out.

Pest control is on-going for every property. 

Ask him what he is doing about it.  He is responsible for getting rid of them, not you, and he may decide to stick you with the bill.   Make sure you have a healthy deposit.  You may want to update the amount.  Don't mention it's because of the bedbugs.  Tell him it just needs to be updated to the current time, 2021.  Check on what he is doing about it and ask for receipts showing what he did. 

Anyone can bring in bedbugs on luggage after staying at an infected motel.  Check with your city's health department about how to get rid of them. 

The tenant would typically be responsible. You clear the infestation & charge them for it. 

also, about 30% of people do not react to bites and might not know they are infested...only if it can be proven a specific tenant started the problem can you charge that tenant ...and even then, not in all states...and it is nearly impossible to prove where the problem started.

NO THEY DO NOT STAY DORMANT FOR YEARS.  At very cold temps (outdoors in cold climates) they have been found to maybe make it 18-20 months).

\NO THIS IS NOT JUST 1 TREATMENT, areas not affected may be okay with 1 preventative treatment, but active areas needs at least 2...often more especially if the tenants don't do their part with cleaning/mattress covers.

NO YOU CANNOT MAKE YOUR TENANTS PAY HALF IN STATES THAT MAKE IT THE LANDLORD'S RESPONSIBILITY.

really? everyone should put their belongings in garbage bags on the law in the sun for a week....that will only work if it happens to be very hot, more likely  things will be stolen.

just because no other tenant admits to bedbugs, doesn't mean no other tenant had them....another tenant could have had bugs, treated their unit and no longer have bugs.

AT MINIMUM, every surrounding apartment must be treated, over, under, next to, across the hall....even in states where the tenant is liable, you need to prove which tenant started it.laws vary.   I HAVE RESEARCHED  THIS..UNLIKE OTHERS.....I know of a situation where a tenant brought in used furniture off a uhaul...and then went to live with his baby mama...every indication is that he started the infestation, but because his unit was actually vacant, it wasn't noticeably affected....a neighboring apartment noticed a problem shortly after.

with an apartment, they can come from any unit and travel through the building, on average half of all units in a building will be infested if the problem is not caught and treated quickly.  A big infestation means they have  been around for a while and have likely spread.

in some states, it is the landlord's legal responsible to pay for the exterminator since it is a building wide issue.

it is often impossible to tell where the problem started.  A person could have brought them back from vacation or had an over night guest bring them in.

individual tenants need to do their part with cleaning, getting bed bug covers for mattresses, etc.  furniture can often be salvaged.  furniture needs to be treated, allowed to dry, and covered for at least a year...most bedbugs don't survive more than 5 months without feeding (longer in colder temps).

and btw, the person with the bed bug issue just got a house...because no matter how often her apartment was treated, the neighbors weren't doing their part and the problem kept coming back.