According to the U.S. Department of State, "a passport is required for all U.S. citizens traveling to Mexico, regardless of the length of stay or the purpose of travel. This includes tourists, business travelers, and students." There are no exceptions to this rule, including for travel to Algodones for medical care.
If you do not have a passport, you can apply for one at a U.S. passport agency or through the U.S. Postal Service. The application process typically takes about 4-6 weeks, so it is important to start the process well in advance of your trip.
In addition to a passport, you may also need to present other documents when you cross the border into Mexico, such as a valid driver's license and proof of auto insurance. If you are traveling with children under the age of 18, you may also need to present a birth certificate or other proof of parentage.
It is important to note that the rules and regulations regarding travel to Mexico can change at any time, so it is always best to check with the U.S. Department of State or a Mexican consulate for the most up-to-date information before you travel.