We will divide our view and analysis of the sources regarding this excellent question into three parts: 1) What causes timtum halev? 2) How severe is exposure to it? 3) Practical recommendations.

What causes timtum halev? The gemara teaches that “sin causes timtum halev.”[3] The idea derives from the spelling of the phrase “v’nitmeitem bam” (you will become defiled by them) in the context of eating sheratzim (crawling creatures),[4] indicating that eating such creatures causes not only tumah (impurity) but also timtum (pollution) of the heart.
There are at least three ways to understand this gemara. These approaches are not mutually exclusive, and may be complementary: A) Acts of sin cause timtum halev irrespective of exposure to a problematic object. This is the simple reading of the gemara[5] and the understanding of the Maharal[6] and Rav Kook.[7] B) The reason that certain foods are forbidden by the Torah is that they impact negatively on the spirit. While this negative effect may be strongest regarding certain specific forbidden foods (e.g., sheratzim), it is true to some degree of forbidden foods in general.[8] C) Forbidden foods were not originally naturally damaging to the spirit, but after the Torah forbade them, they became so.
There are practical differences between these approaches. Importantly, some prohibitions seem to lack a naturally damaging element: 1) Performing forbidden actions in which one does not ingest a food; 2) Eating foods that are forbidden based solely on Rabbinic law. HaShem apparently did not create these foods as ones that cause timtum (for if He had, they would have been forbidden on a Torah level[9]), but the Rabbis nonetheless forbade them due to various halachic concerns, especially due to the possibility of their being confused with innately forbidden foods; 3) Eating foods that are forbidden (at least to specific people) because of the foods’ special holiness (e.g., teruma, certain korbanot); 4. Eating foods that are forbidden only at certain times (e.g., food on Yom Kippur, chametz on Pesach); 5. Eating foods that are forbidden for a circumstantial moral reason (e.g., a mother animal and her child that were slaughtered on the same day; eiver min hachai[10]). According to approach A, timtum halev would be expected in these cases, whereas according to approach B it apparently would not be. (Following approach C, the question of timtum could be debated in several of these cases.)
Conversely, there are some cases in which prohibited foods enter a person’s body without moral culpability, such that approach A would not apply whereas approaches B and C would: 1) The person eating followed halachic rules, which resulted in his ingesting the forbidden food (e.g., an animal had blemishes that make it a treifa, but we are not required to check for that blemish; it was permitted due to bitul); 2) The eater is not forbidden to eat the food (he is a non-Jew, a small child, or severely mentally disabled); 3) The person was compelled to eat the normally prohibited food to save his life; 4) The substance entered the body in a way other than eating.
We will begin with a small sampling of the many sources that present different views on some of these matters.
Chashukei Chemed[11] cites a machloket regarding whether eating forbidden food due to an unavoidable mistake creates negative spiritual effects (Rav Pe’alim says no; Rama MiPanu says yes[12]). One explanation for the opinion that there is no timtum halev in this case is approach A, which maintains that the source of the timtum is the sin involved. However, one could also suggest that HaShem would intervene to rectify the spirit of an individual who followed Halacha properly.[13]
The Netziv[14] writes that the reason it is better to shecht an animal on Shabbat for a dangerously sick person than to have him eat non-kosher meat, even though the former action is apparently a more severe prohibition, is that the latter option causes timtum. This assumes that timtum exists even in the absence of wrongdoing. However, we do not find Rishonim giving this explanation,[15] which would seem to demonstrate the opposite.

How severe is exposure to timtum halev? Many halachot assume that even if timtum halev is caused without wrongdoing, it is not a serious normative factor. One is not required to stop a child from eating non-kosher food,[16] even though we would certainly stop him if we were to see him ingesting poison. (It is difficult to claim that the lack of a requirement to stop the child from eating non-kosher food is “only” on a halachic level, whereas one must still save the child due to timtum.[17])
If 49 pieces of treif meat were to become mixed in with 50 pieces of kosher meat, bitul (sometimes) enables the eating of all the pieces.[18] (Some have the stringency of first removing some meat to avoid the appearance of impropriety.[19]) Similarly, if treif gravy falls into a larger amount of kosher food and thereby lowers the kosher food’s quality of taste, it is permitted to eat the combination.[20] In neither case do poskim raise the issue of timtum halev. In fact, there is a machloket if it is permissible to refuse to eat food that is permitted based on bitul.[21] If Reuven sold non-kosher food to Shimon, who then ate it, Shimon is entitled to a full refund if the food was forbidden from the Torah, but no refund if it was only Rabbinically forbidden.[22] The main reasons given for this halacha[23] do not raise timtum halev as a factor in either case. Thus, it seems clear that on a normative level, when the eater lacks guilt, there is no timtum halev, or at least the timtum is halachically insignificant.
Nevertheless, there are normative halachic sources that warn about timtum halev. The Rama[24] writes that one who has a choice in the matter should not have a baby nurse from a non-Jewish or not kashrut-observant wet nurse. This is based on the Rashba,[25] who holds that a non-Jewish woman’s milk is kosher, but it is pious to avoid giving it to a baby due to concern for the baby’s future spiritual health. Along the same lines, the Chatam Sofer,[26] after halachically justifying sending a shoteh child to a non-Jewish center at which he had the best chance at improved mental health, nevertheless advised not to send him there in practice, due to timtum halev. (Many argue with the Chatam Sofer’s advice or limit its scope.) Even if these are “extra-halachic” pieces of advice, why do they come up when they do?
One cannot always reconcile exceptional rulings with the rules. However, these cases, especially that of the Rama, have unique factors. A baby’s basic sustenance on an ongoing basis is from nursing, during a crucial point of development in which he lacks performance of mitzvot and has few other things to counteract timtum halev.[27] (Yabia Omer[28] does not accept this distinction, as he views the Rama’s ruling as reason to prefer, if possible, accepting a blood transfusion from a kosher-eating Jew.)

Practical Recommendations: While some compare eating non-kosher food to poison,[29] one dose of such food apparently does not “kill” one’s spiritual soul. Rather, the more one is exposed, the worse it is for the person. Several components cause timtum halev and other similar concepts (e.g., ruach ra’ah in food touched before netilat yadayim[30]), and many things rectify problems of the spirit. The average person should trust Halacha to factor in this element in a balanced manner, and he need not personally factor timtum halev into his halachic decisions; concern for possible sin is serious enough. One who strives for spiritual near-perfection might need to factor in even the finest points, but this response is not geared for such unique people.