With slow food throughout, the intestinal valve is a favorite lodging site for internal parasites, most notably tapeworms. They spend all of their lives not just in one species of shark but only in a single section of its valve. The dusky smoothhound or smooth dogfish, for example, has a spiral valve with eight chambers. The first four chambers are home to different, chamber-specific species of tapeworm. By the time food has reached the fifth chamber, much of the nutrients have been removed, so the rest of the chambers are unoccupied.

Parasites are a fact of shark's lives. There are parasitic copepods that saunter in and out of their gills and nasal passages or attach themselves firmly to the edges of their fins, and marine leeches that make a nuisance of themselves around the cloacal area, but surprisingly, sharks are usually untroubled by serious natural diseases.

Sharks are thought to be the first ones to develop an effective immune system. Their bodies have survived several diseases and disasters in past several hundred million years. They have evolved through it. For instance, their blood has high levels of immuno-globulin that is ready to destroy any invading particle at any given in their body. Immune cells develop in spleen, thymus, gonads, oesophagus rather than just the bone marrow as with other mammals. They circulate in the blood, ready to go to work with hardly any lag between infection and response. As a result, sharks can live long and relatively healthy lives if left unmolested, but they are slow to grow and mature. Tagging studies in the northwest Atlantic have shown that the sandbar shark, for example, may live for 40 to 50 years but does not reach sexual maturity until it is 30 years old.