why a disease that was once on the decline might re-emerge?

People could lose their immunity or new strains of the disease may develop.

due to the storage of hypnozoits and sporozoits stages of parasites(dorment stage)

It could be new strains that mutate so that they are not recognizable by the antibodies we developed to the previous strain (the flu does this year after year). Another reason is some people, though there isn't ANY evidence supporting this, believe that vaccinations give autism. Therefore there is a high percentage of babies not vaccinated for diseases we once thought were eradicated in the US. This sets the stage for communicability between these unvaccinated children in deadly diseases such as smallpox (a crime I personally believe should be gross negligence on part of the parents).

A disease that was once on the decline might re-emerge due to a variety of reasons. Understanding these reasons can help us prevent and address such situations effectively. Here are a few possible explanations:

1. Reduced vaccination rates: Vaccines play a crucial role in controlling and eradicating diseases. If the vaccination rates drop, particularly in a specific region or community, the disease can find opportunities to re-emerge. This may happen due to people's vaccine hesitancy, lack of access to healthcare, or misinformation about vaccinations.

To tackle this, ensuring widespread access to vaccines, promoting vaccine literacy, and addressing concerns or misconceptions about vaccination are essential.

2. Pathogen mutation: Pathogens, such as viruses or bacteria, can undergo genetic changes over time, leading to the emergence of new strains or variants. If a mutated strain becomes more virulent, resistant to available treatments, or capable of evading the immune system, it can cause a resurgence of the disease.

Monitoring and surveillance systems that track genetic variations of pathogens are crucial for detecting and responding to such changes promptly.

3. Global travel and migration: With increased international travel and migration, diseases can quickly spread across geographical boundaries. If a disease is present in one region but under control, travelers or migrants from areas with a higher disease burden can reintroduce the infection to a population with lower immunity, causing a re-emergence.

Strict border controls, effective screening measures, and international cooperation in data sharing and surveillance can help prevent the spread of diseases through travel and migration.

4. Environmental changes: Environmental factors can influence the prevalence of certain diseases. For example, changes in climate patterns, deforestation, urbanization, or population displacement due to natural disasters can create favorable conditions for disease-carrying vectors (e.g., mosquitoes) or disrupt healthcare infrastructure, leading to disease resurgence.

Investing in environmental conservation, implementing robust public health measures in affected regions, and adapting healthcare systems to address climate change impacts are crucial for preventing disease re-emergence.

In summary, diseases that were once declining may re-emerge due to factors such as reduced vaccination rates, pathogen mutation, global travel and migration, and environmental changes. Addressing these underlying causes and maintaining consistent surveillance and prevention efforts are key to preventing the resurgence of such diseases.