Parasites have a eukaryotic cell structure similar to that of human cells and are divided into two types: helminths and protozoa.

• Helminths are flatworms and roundworms that can be seen with the naked eye. Some helminths mature from larvae into adults in the GI tract (making humans the definitive host) and usually cause mild disease. In contrast, the larval stage of some helminths penetrates the intestine; migrates through tissue; invades organs, where larvae mature into adults (making humans an intermediate host); and may cause severe disease.
  • Except for Strongyloides and Capillaria, all helminths require stages in other hosts to complete their life cycle. Thus increases in the burden of infection require repeated exogenous infections (e.g., residence in an endemic region).
• Protozoa are microscopic single-celled organisms that, similar to bacteria, multiply within the human body and cause overwhelming infections.
  • Naïve pts with their first protozoal infection usually are the most severely affected because partial immunity often limits the number of parasites during recurrent infections.
• Table 109-1 Parasitic Infections, by Organ System and Signs/Symptoms breaks down the symptoms of major parasitic infections by organ system and geographic distribution, with comments on clinical and epidemiologic associations.