A key aspect to keep in mind is medieval-era medicine was far different in conception and practice than the modern medical practices which would develop centuries after Contact. A confluence of natural and supernatural forces as causative of disease was common among both the Europeans and Americans, and even drawing a distinction between the two is a somewhat anachronistic dichotomy. López Austin^1 discusses the etiology of illness as

including and often intertwining two types of causes: those that we would call natural -- excesses, accidents, deficiencies, exposure to sudden temperature changes, contagions, and the like -- and those caused by the intervention of nonhuman beings or of human beings with more than normal powers. For example, a native could think that his rheumatic problems came from the supreme will of Titlacahuan, from the punishment sent by the Tlalolque for not having performed a certain rite, from direct attack by a being who inhabited a certain spring, and from prolonged chilling in cold water; the native would not consider it all as a confluence of diverse causes but as a complex. (pp. 216-217)

Illness could therefore be both a natural process, with mundane causes, while simultaneously being the work of supernatural forces. This is not particular to Mesoamerica, but the exoticization of Indigenous peoples often means the more spiritual elements are emphasized over the practical. Gout (coacihuitztli), for instance, can be approached as a local inflammation stimulated by cold winds and particularly wind from caves, to be treated with a topical application of tobacco and axin. The more eye-catching approach, however, is viewing the disease as caused by the Tlaloc, to be treated with rituals and by smearing a paste of hallucinogens on the affected area as sort of a way of creating a spiritual link between the affected area and the causative god. Likewise, xoxalli an affliction causing "cold feet" and swelling of the feet and ankles, was caused by too much walking, a perfectly cromulent natural etiology. This explanation, however, leaves out that walking was thought to stimulate the tonalli (part of the tripartite soul) which then took heat from other parts of the body. As with gout, the solution was to rub the area with nicotine and axin, considered "hot" substances which would counteract the "cold" of gout and xoxalli.

There's nothing wrong with talking about the supernatural aspects of Nahua medicine, so long as we keep in mind that the strict distinction between the natural and mystical is one being imposed from the outside. Hot/Cold dichotomies, appeals to the gods, and botanical cures which could incorporate both sympathetic elements alongside curative ingredients were practical approaches to disease in a world where the ailments could be simultaneously seen as being smitten by the gods and as a result of not dressing warmly enough. Ortiz de Montellano^2 suggests a helpful approach to think of diseases having both "ultimate" and "proximate" causes, explaining that

Natural etiological systems focus mainly on proximate causes and the process of disease. Supernatural and magical systems are more concerned with ultimate cause, that is, why a particular disease affects a particular person. (p. 130)

Spanish concepts of disease were no less entangled in a combination of practical and spiritual. Díaz del Castillo^3 speaks of rinsing wounds and wrapping them with linen, but he also talks about smearing the wounds with fat rendered from a human corpse and being bled to cure an instance when he had a fever and was vomiting blood. Ultimately, of course, he ascribes his recovery from both sickness and injury to the grace of God.

When the Spanish and Aztecs met each other they were both operating with similar medical schema which intermingled the mundane and the divine. As such, the Spanish were fine with adopting practical remedies. Likewise, Newson^4 notes that while there were university-trained physicians in Spain, there was an array of healers outside that class including “midwives, bonesetters, and dentists, and others who treated hernias, cataracts, or extracted bladder stones” (p.373) who drew upon herbology as well as an array of folk traditions and superstitions which could trace their origins to pre-Christian or Arab sources. The Spanish in the Americas were fresh off the Reconquista and were well used to incorporating non-Christian elements into their culture (while obviously excluding other aspects).

The flexibility of the Spanish healthcare system is essential because, even in Spain, licensed university-educated physicians were scarce, leading Newson to note

it is clear that the majority of the population relied on medical care provided by hospitals run by the religious orders and on a range of popular healers or curanderos, many of whose practices were embedded in magico-religious beliefs. In fact even the elite, including licensed physicians, consulted popular healers (p. 369)

The shortage of “official” physicians in Mexico was even more acute, with many of the medical practitioners falling into the range of the “empirics” described above and with even some indigenous practitioners given authorization to practice.

It’s important to note here the power dynamics of Mexico under Spanish rule. While nominally in control, the Spanish authorities were a tiny minority in a huge and populous country. The authority of the Spanish government relied upon the cooperation of Indigenous nobles and other elites, while also being careful not to tread too harshly on native customs. I’ve written about this in the context of sacrifice and “idolatry” in a previous comment, but a key point is that the Spanish authorities were generally tolerant of traditional practices continuing, so long as the practitioner outwardly expressed Christian beliefs.

To cite Newson again, she argues that the dearth of European medical professionals in the Americas essentially forced the Spanish to tolerate native experts both as a matter of practicality and to avoid creating a locus of mass resistance. Far better to co-opt and adapt the vast knowledge of medica materia present in the Nahua ticitl and other practitioners.

This is not to suggest the Spanish were eager to make sacrifices to Tlaloc to cure their colds. Huguet-Termes^5 is not alone in noting the Spanish, even as they readily integrated the natural sciences of the Americas into their medical corpus, they did so by rejecting the “magico-religious context” of the materials in favor of incorporating the various cures into the prevailing Galenic humoral system. For instance, Francisco Hernandez, the court physician sent to compile a work on the medical uses of the plants, animals, and minerals of Mexico, classified his findings in terms of humoral properties, which at times put him in agreement with Indigenous knowledge of the effects of a plant, even as he disagreed with its mechanism of action.

An example of the Hernandez’s Galenic worldview awkwardly bumping up against American practice can be seen with his entry on the “quauhtlepatli, or fire tree.” Hernandez writes^6

It’s milky sap is astringent; nevertheless, the Indians say that, taken in the amount of oboli, it very easily evacuates the phlegmatic humors of those suffering from cachexia, the French disease, or dropsy, especially if the illness proceeds from a cold cause. (p. 123)

The disagreement is subtle, but in the Galenic view, astringent materials are supposed to dry up excess secretions. The quauhtlepatli, which Hernandez identifies as a type of rhododendron, instead “evacuates” the phlegm, in the form of vomiting, salivation, and diarrhea. Such disjunction between what medicinal plants were used for and what Hernandez thought they should be used for, or how they should be used, led him to write of the *ticitl” that

even when they have a marvelous array of healthful herbs to choose from, they do not know how to use them properly, nor exploit their real value. (p. 78)

Not every compilation of Mexican medical knowledge was so steep in humoral chauvinism, however, and Sahagún’s General History of the Things of New Spain remains a key source for cataloging the botanical knowledge of the Nahuas. López Austin and Ortiz de Montellano both note, however, that Sahagún, a Spanish friar, appears to have deliberately stripped his entries of their Nahua religious contexts, preserving only the naturalistic elements. Ortiz de Montellano notes that an earlier draft of Sahagún’s work, the Primeros Memoriales includes entries on certain ailments classified as atonahuiztli (aquatic fever) which included as treatments taking hallucinogens as a pathway to the gods. The later, final version of Sahagún’s work eliminated these entries, along with entries on death in childbirth and by lightning strike which contained overt religious elements.

The Spanish, in other words, were happy to incorporate Mesoamerican knowledge of herbs and other medicinal materials into their own medical corpus. Colonial authorities were also able to tolerate the continued practice of Indigneous physicians for a number of reasons. The result was syncretism on both sides, with Galenic and Mesoamerican blending together. At the same time, Colonial authorities insisted on a strict outward affirmation of a Christian worldview, which meant that the natural science aspect of Mesoamerican medicine was preserved even as the spiritual connections were minimized or transferred onto Christianity.