We've all got some of that. Way back when our ancestors lived in isolated communities and didn't move around all that much, the choice of mates was a bit limited. Sooner or later, someone had to marry someone who was related.
Populations isolated either by geography or by religion tend to have a bit more inbreeding. Geneticists have identified about 93 genetically-carried diseases common among the Jewish people and 72 for the Amish. This is not to say that every Jewish person or every Amish person has or even carries a disease, but it's a much higher number than you'd find in larger groups, or that belonging to another group means you'll never get the disease. Some scientists think multiple sclerosis originated with the Vikings and that they spread it around.
One result of the Reformation was substitution of the Catholic rules on cousin marriages with the rule found in Leviticus on the subject -- the ones that won't let a man marry his brother's widow, but doesn't say squat about marrying your first cousin. Up till the 1850s, first cousin marriages were quite common, both in the United States and elsewhere -- Charles Darwin married a first cousin, for example.
I am not a geneticist, a scientist, or anything of the sort. I just keep my eyes open. I know people who are the products of first-cousin marriages who are perfectly normal. In fact, I know someone whose ancestry includes a father-daughter relationship (several generations back) who is perfectly normal. My unscientific opinion (which ain't worth much, admittedly, but is backed up by others who know more) is that the occasional episode of cousin marriage in one's family tree isn't necessarily a bad thing. Close cousin marriages do increase the chances somewhat of the descendants inheriting a birth defect, but not by much under normal circumstances. It's when you have cousins marrying cousins and their children marrying each other, over and over again, like the royal families of Europe did, that you become virtually guaranteed of running into problems.
We can talk about George III's madness, caused by inherited porphyria, or the hemophilia that runs among the descendants of Queen Victoria (including the last Czarevitch of Russia), but that's nothing compared to the disaster that was King Carlos II (1661-1700) of Spain, known as Carlos the Bewitched. As his family tree shows, his maternal grandfather and paternal grandmother were brother and sister. So were his paternal grandfather and his maternal grandmother. Now a first cousin marriage is considered bad enough, but his parents were doubly first cousins. In other words, genetically speaking, his parents were virtually brother and sister, having the same grandparents on both sides of their families. Furthermore, they, or at least a lot of them, were members of the Habsburg clan themselves. (The Habsburg power must be the only reason the Pope ever gave the dispensation for this this extremely ill-advised marriage.) Carlos II, the only one of the five children born to his parents to survive, was mentally retarded and physically deformed. For example, he had a wildly exaggerated form of the Habsburg jaw, which meant that it was impossible for him to chew his food. Fortunately for the gene pool, he could not father any children, either -- goodness knows he tried. (His wife was the niece of the King of France -- not a Habsburg -- did they finally get the hint? The poor woman was married to this walking genetic defect because he was, after all, still King of Spain.) Unfortunately for Spain, his death led to the War of Spanish Succession.
What makes the very existence of Carlos II even more astonishing is that it had happened at least once before in the Habsburg family -- Don Carlos, the son of Philip (Felipe) II by his first wife and double first cousin, Maria Manuela of Portugual, had only six great-great-grandparents instead of the usual 16 (two of his great-grandmothers were sisters). He was physically deformed, mentally retarded, and prone to violence. When he died, it was thought that his father had had him poisoned, and nobody seemed very sorry about the fact (except his stepmother). You would think the Habsburgs learned a lesson from Don Carlos, but no ... in fact, not long after Don Carlos died, Philip II's third wife, Elisabeth died, and he turned around and married his niece, the Archduchess Anna; they were eventually the great-grandparents, twice over, of Carlos II. (About the only good thing you can say for Carlos II is that he wasn't violent, as Don Carlos had been.)
Here's another one about first cousin marriages.
SECOND COUSINS: Johannes Fahrenbach and Johanna Kamp
SECOND COUSINS: Johannes Michaelis DeGuenther/DeKinder II and Maria Catharine Faust.
FOURTH COUSINS: Angele Archambault and Narcisse Janot dit Lachapelle.
SEVENTH COUSINS: Georg Anton Betschart and Brigitta Betschart.
I have, furthermore, multiple Joneses, Evanses, Hansens, Suters, F–hns, Wiltshires and Janots in my direct line that have not been traced back to a common ancestor, and wouldn't be surprised to find a few more loops. But don't call me Inbred Jed. :)