I believe the United States having open border policies is a disaster waiting to happen. If we have open borders, then the United States will be open towards not only disease, corruption, etc, but is also open towards attacks, war, militia attacks, and much more. If we open our borders, we'll also be giving illegal immigrants who aren't registered opportunities to commit crimes and even if they are kicked out, they will be able to re enter and continue. This would also create a huge economic crisis. Illegal immigrants don't have to pay taxes, healthcare, insurance, etc.
Illegal immigrants are people who enter into a country ILLEGALLY. Open Borders means that people can go through borders without any restrictions from stopping them. Also, about disease, that has not been proved because international airports is an hot spot for disease since millions of flights are made across the world. Also, it is very unlikely that there be war because of open borders. The United States spends more money than any other nation on earth and has more troops than any other country except People's Republic of China. Also, it is part of NATO, which in the treaty says that an attack on one country in NATO, is an attack on countries in NATO, which includes the United Kingdom, France, Germany, Turkey, and Canada. Therefore, open borders have nothing bad about them.
We'd make ourselves prone to war. Although we are in NATO, it would be a strain on other countries to have to assist us if someone undocumented from another country attacked us. We also have plenty of illegal activities going on with the borders up. The number would spike up so much higher if drug dealers, human traffickers, etc were allowed to just enter and go as they please! An open border would just make the US a criminal hangout for many.
You see, if Mexico and America has a criminal extradition treaty, which is one country transfers a suspected or convicted criminal to another country, therefore allowing both nations to take criminals (drug cartel leaders, human trafficking leaders, organized crime leaders) to back to the other country to face justice. Not only that, the police force of the United States is 765,000 officers, not including SWAT and FBI members. Then there Interpol, an international police organization that works against some of the thing you mention, including human and drug trafficking, weapons smuggling, organized crime (mafias and drug cartels are examples), and corruption. Not only that, every country on mainland North America are part of Interpol.