More than 1,000 cases of measles have been reported in more than two dozen states this year, putting the U.S. at risk of bringing back a deadly disease we once eradicated. The Centers for Disease Control (CDC) states that the disease is so contagious that if “one person has it, up to 90 percent of the people close to that person who are not immune will also become infected.”

Measles — a highly infectious, vaccine-preventable disease — has spread rapidly in the U.S. this year in part because some parents have recklessly chosen to forgo vaccines for their children for no good reason. This poses a serious public health threat. Parents have a responsibility to vaccinate their children both for their own protection, and for the protection of others.

The rise in the number of measles cases is particularly dangerous for unprotected populations unable to get immunized due to age or health reasons. A baby, for example, cannot receive the measles vaccine until one year of age.

According to the CDC, the recent measles outbreaks in the U.S. have been linked to unvaccinated U.S. travelers who brought measles back from other countries where there are ongoing measles outbreaks. This should serve as a warning to us about loopholes in our vaccination requirements for immigrants and visitors, which may increase the likelihood of future infections of measles or other diseases.

The CDC requires the more than one million people who legally immigrate to the U.S. each year to be vaccinated. Nonimmigrant (temporary) visa holders, on the other hand, are not required to comply with this rule. While officials are required to perform medical evaluations on immigrants who enter the U.S. illegally, these individuals are not subject the same proof of vaccination as immigrants who arrive legally.

The growing humanitarian crisis at the southern border makes this situation even more worrisome. The number of migrants arriving at the southern border with the intent of entering the U.S. illegally continues to surge. In May, more than 132,000 individuals were caught attempting to illegally cross the border.

These increasing numbers have led to a shortage of border patrol staff, services, and facilities, all of which are needed for newcomers who need processing, shelter and medical care. There is often an initial period before symptoms appear, meaning an infected individual may not get the medical treatment he or she needs before being released into the community.

This is unacceptable. This puts Americans, as well as immigrants, at risk.

To further complicate matters, there are infectious diseases like Zika, Ebola, tuberculosis (TB) — the world’s most deadly infectious disease, according to the World Health Organization — as well as a new virus affecting children that mimics polio that do not have licensed vaccines available. These are all primary concerns when considering that more than 60,000 migrants illegally slip into the U.S. each year undetected.

Legal immigrants must comply with our laws requiring immunization to prevent diseases such as measles, mumps, and polio. However, there is no way to enforce that for those who cross our borders or remain in our country illegally. For public safety, the CDC should act to ensure that anyone who plans to live in the U.S. provide proof of vaccination. In conjunction with a vaccine requirement, action must also be taken to immediately provide the funding needed to secure the border.

The government can also take additional steps to encourage Americans to vaccinate against measles and other illnesses. The Measles Mumps Rubella (MMR) vaccine is safe and effective, and has been available since 1963.

Outbreaks raise serious health and public safety concerns for all. For the sake of today’s U.S. citizens and also for the sake of people around the world who hope to one day call America home, we must ensure that we are adequately protected from the spread of infectious disease.