**Core Concept**
Autoinfection refers to the re-infection of an individual by the same pathogen, often through a continuous cycle of infection and re-infection. This mode of transmission is particularly relevant in the context of parasitic infections, such as hookworms, where the larvae can penetrate the skin and re-infect the host.

**Why the Correct Answer is Right**
Autoinfection typically occurs through a breakdown in the host's immune response or an alteration in the normal life cycle of the pathogen. In the case of hookworms, the larvae can penetrate the skin and migrate through the body, eventually reaching the lungs and being coughed up and swallowed, only to re-infect the host through the gastrointestinal tract. This cycle can continue indefinitely, making autoinfection a significant mode of transmission for these parasites.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe autoinfection. While it is a mode of transmission, it is not the only one, and it is not specific to the context of parasitic infections.
**Option B:** This option is incorrect because it refers to a different mode of transmission altogether. Direct contact transmission typically involves the direct transfer of pathogens from one individual to another through physical contact, often through skin-to-skin contact or through contaminated objects.
**Option C:** This option is incorrect because it refers to a different type of infection altogether. Concomitant infection refers to the simultaneous presence of two or more pathogens in an individual, which is distinct from autoinfection.

**Clinical Pearl / High-Yield Fact**
Autoinfection is a key consideration in the management of parasitic infections, particularly those caused by hookworms and other nematodes. It highlights the importance of thorough patient education and follow-up to prevent re-infection and ensure effective treatment outcomes.

**Correct Answer: D. Hookworms**