The notion that medicines derived from plants depend for their action on active principles present has to be modified in view of the findings that there are, in many cases, adjuvant substances in the plant which enhance the activity of the components actually responsible for the effect. This synergy may involve protection of an active substance from degradation by enzymes; it may facilitate transport across barriers such as cell and organelle walls, it may overcome multi-drug resistance mechanisms or provide other signals to the host's cells that result in higher efficacy of the crude drug when compared with isolated components. The many plant substances that stimulate the immune system, often at very low doses, have not been reviewed as this is not strictly speaking synergy. Some of the evidence for the phenomena described is reviewed and its bearing on phytotherapy commented.

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This study investigated a model to assess the role of climate fluctuations on dengue (DENV) dynamics from 2010 to 2019 in four Brazilian municipalities. The proposed transmission model was based on a preexisting SEI-SIR model, but also incorporates the vector vertical transmission and the vector’s egg compartment, thus allowing rainfall to be introduced to modulate egg-hatching. Temperature and rainfall satellite data throughout the decade were used as climatic model inputs. A sensitivity analysis was performed to understand the role of each parameter. The model-simulated scenario was compared to the observed dengue incidence and the findings indicate that the model was able to capture the observed seasonal dengue incidence pattern with good accuracy until 2016, although higher deviations were observed from 2016 to 2019. The results further demonstrate that vertical transmission fluctuations can affect attack transmission rates and patterns, suggesting the need to investigate the contribution of vertical transmission to dengue transmission dynamics in future assessments. The improved understanding of the relationship between different environment variables and dengue transmission achieved by the proposed model can contribute to public health policies regarding mosquito-borne diseases.