Many of the homes are in run down shape and yes as i looked i saw it all from beer cans every where and not working toilets to houses completely not level as much as 5 inches... to bongs and drugs left every where.

Once again, this isn't a problem in itself as some drainage can help alleviate the problem and generally isn't terribly expensive but if you are holding long term, I would ensure the issue is resolved.  

Turned out to be owners son whom was a drug addict.

When I searched BP - I found great horror storries of nude tenants, drug equipment, dead dogs etc. so I know it can be done.

You do not say where this is at or how many units you own.Some of those areas in down town are just crap holes and that type of thing comes with the territory.If you have a large complex you could put up a gate with card access to keep the bums, trouble makers, and drug dealers from loitering etc. around your place and give the tenants piece of mind.Have you set up security cameras and street lights??

If they don't move it's because they are dealing drugs and don't want to give up their "store front".

While there are no known drug/antiviral treatments for those that are infected, the virus itself can be killed with simple disinfectants such as alcohol and bleach. 

An investor I know is under contract to sell a rental house of his for a national drug store and a bank location. 

Unfortunately, not all the illegals are good hearted people with good intentions and many are cartels that traffic drugs and sex, gangs, spies, terrorists, etc.

However; having a home built on the lot yourself would alleviate the risk considerably.