Trypanosoma cruzi and the Leishmania donovani complex are parasitic protozoa that, respectively, cause Chagas disease in the Americas, and visceral leishmaniasis, predominantly in South Asia, East Africa, and Brazil. T. cruzi is divided into the lineages TcI-TcVI. The relationship between infecting lineage(s) and spectrum of clinical presentations remains poorly understood. This project developed lineage-specific serology to identify an individual’s history of lineage infection. A high level of polymorphism in the surface mucin TSSA was identified, and lineage-specific synthetic peptides based on this diversity were applied here in ELISA with chagasic sera from endemic countries. Peptide TSSApep-II/V/VI, based on a sequence common to those lineages, was widely recognised by sera from Southern Cone countries, and also unexpectedly by four samples from Ecuador; TSSApep-V/VI, which differs by a single amino acid from TSSApep-II/V/VI, was also recognised in these regions. A single TSSApep-IV reaction was seen in both Colombia and Venezuela. However, TSSApep-I was rarely and weakly recognised among the serum panel. Among the Brazilian patients, a much higher proportion of TSSApep-II/V/VI responders had ECG abnormailities than non-responders (38% vs. 17%, p<0.0001). Rapid diagnostic tests for L. donovani complex infection based on rK39 antigen have lower sensitivity in East Africa compared to South Asia. The homologous sequences of rK39, and of another proposed diagnostic antigen HASPB, were amplified from a panel of East African L. donovani strains, and compared to published sequences, revealing significant diversity from rK39 and South Asian sequences, and non-canonical combinations of HASPB repeats. Cohorts of Indian and Sudanese VL patients were assayed by ELISA for anti- Leishmania IgG levels. There was an overall 46.8 – 61.7 fold lower response in the Sudanese cohort, as calculated by mean reciprocal log10t50 titres, regardless of antigen source, patient gender or age. An investigation into the association of IgG subclass reactivity with VL clinical status revealed significantly elevated IgG1 levels in patients with active (pre-treatment) VL and those with post-therapy relapse compared to those deemed to be cured. A novel prototype rapid immunochromatographic test to detect IgG1 gave > 80% of relapsed VL patients as IgG1 positive, and 80% of cured patients as IgG1 negative.