Mexico is on the southern border of the USA. The two countries share a 2,000 mile border. This border is essentially unprotected and illegal immigrants come over the border all the time. Most of these immigrants come for work and a better life. Some are escaping abuse, etc. Some are bringing drugs over the border. The list is endless. That key fact is that these people are enetringthe USA illegally and then they may or may not: (1) ever leave, (2) commit criems in the USA, (32) get US Givt,. benefits while here, etc.
Although we benefit from their presense here because they perform jobs that may otherwise not be filled (low wage jobs), the total cost to the USA citizenry is high. Emploters lie Walmart hire these people and then since they only get part-time jobs or don't make enough money to support a family, they must then get government support via food stamps and other programs. That means that the citizenry are underwriting employers like Walmart who do not pay their employees enough.
Central American Children - Starting in 2013-2014 a flood of Central American children came over the USA-Mexico border seeking refugee status. The USA allowed them in due to UN rules concerning refugees (if a refugee comes form at least 1 country away from the USA, the USA must allow them in. We have allowed millions of children in and they will become USA citizens.
Illnesses - many of tehse children had ilnnesses and were allowed to come across the border and then were shipped out at US expense to locations throughout the USA where they either met up with family embers, or were taken in by reigious organizations, etc. However, these children may never have been innoculated against serious diseases in their originating countries. In late 2014 and into 2015 we hac measles and whooping cough breakouts in the USA and the US Govt and Big Pharma were quick to recommend innoculating all of our children against these things even though the Govt was responsible for spreading the risk around the US via these Central American kids.