Abstract. A 50% infectious dose (ID 50 ) of 132 Cryptosporidium parvum oocysts was previously determined in serologically negative individuals (ELISA). In this study, 17 healthy adults with pre-existing anti-C. parvum serum IgG were challenged with 500-50,000 oocysts. Infection and diarrhea were associated with the higher challenge doses. The ID 50 was 1,880 oocysts, Ͼ 20-fold higher than in seronegative volunteers. Fecal oocysts were detected in only seven (53.8%) of 13 individuals with clinical cryptosporidiosis, indicating that the host response may effectively decrease the number of oocysts produced. Subjects with the highest absorbances prior to challenge had little to no increase in IgG following challenge, whereas volunteers with lower reactivities showed significant postchallenge increases. This suggests that an upper limit of serum IgG was present in some subjects, while others were further stimulated by an additional exposure. These data indicate that prior exposure to C. parvum provides protection from infection and illness at low oocyst doses.Cryptosporidium parvum is a recognized cause of diarrheal illness in waterborne outbreaks [1][2][3] and in individuals with human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS). 4 The increased awareness of cryptosporidiosis in the general population and in specialized settings, such as day care centers, as well as the seriousness of the disease in immunosuppressed individuals have earned Cryptosporidium a place on the list of emerging diseases that are a threat to the public health. 5 Furthermore, no effective chemotherapeutic agent has been demonstrated to prevent or cure C. parvum infections. Thus, there is considerable interest in the development of immunotherapies designed to limit infection. To rationally design such interventions, it is important to understand the major immunologic mechanisms that are operative in self-limited disease. Animals 6-8 and humans 9 with intact immune systems are typically capable of clearing the parasite within 1-3 weeks after infection. Several studies with experimental animal models have demonstrated that most species are refractory to a second oocyst challenge. 7,8 Indeed, adult animals exposed to the parasite for the first time are relatively resistant to infection and often have only a transient period of oocyst shedding with few to no symptoms. In contrast, healthy adult volunteers can experience infections and diarrheal illnesses when oocyst challenges are at least one year apart. 10 Many studies in animals 11-15 and humans [16][17][18][19] have documented the presence of antibodies to C. parvum infection. However, these investigations in humans were carried out in serologic surveys or in select persons experiencing diarrhea and only after the infections were diagnosed. Thus, the observations were necessarily limited by the lack of information on the immune status and exposure history prior to the exposure. Also, differences among the studies in antigen preparations used in the testing and definiti...

The initial acquisition of detectable H. pylori infection occurred at a rate of 20% per year among Pasitos Cohort children from birth to 24 months of age. A key finding, with implications for clinical, community health, and research settings, is that most of these infections did not persist. The transient nature of early H. pylori infection should be considered when designing research or contemplating therapeutic intervention for this age group.

Background: Compared to smoking cigarettes, use of Western smokeless tobacco (ST) products is associated with a very small risk of life-threatening disease (with estimates in the range of a few percent of the risk from smoking, or even less). This means that smokers can realize substantial health benefits by switching to ST, an obvious substitute. But consumers and policy makers have little chance of learning that ST is much less dangerous than smoking because popular information provided by experts and advocates overstates the health risks from ST relative to cigarettes.

Self-rated future health is an independent, robust predictor of mortality. It is as predictive of subsequent mortality in older adults as the standard measure of self-rated current health. Furthermore, a measure that combines self-reports of current health with future health was most useful in the identification of older adults with the highest mortality rates. Thus, the combined measure of current and future health may be most useful in practice, in distinguishing the differential mortality rates among persons reporting fair or poor self-rated current overall health.

The underlying complexity in population-disease dynamics requires population-specific descriptions of trends using multiple methods to provide an in-depth analysis while simultaneously allowing for necessary statistical adjustments as well as identification of interactions. More thorough descriptions of the population-specific general trends in relation to changes in the population structure (age-period-cohort) enable better prevention and health care policy planning, and further, the descriptions enable hypothesis generation regarding causes of population-specific disease patterns.