1. Spanish Flu: Death toll. https://en.wikipedia.org/wiki/Spanish_flu.
2. Spanish Flu: GDP loss. https://www.encyclopedie-environnement.org/en/health/viral-pandemics-of-the-modern-era/#:~:text=Viral%20pandemics%20of%20the%20modern%20era%20(1918%20to%20the%20present,%2C%20SARS%2DCoV%2D2.&text=The%20Mexican%20swine%20flu%20(Influenza,H1N1%20virus)%202009%2D2010.
3. HIV/AIDS epidemic https://en.wikipedia.org/wiki/Epidemiology_of_HIV/AIDS.
4. World Health Organization (WHO) website, Small-pox: https://www.who.int/csr/disease/smallpox/vaccines/en/.
5. COVID-19 cases: Global count. https://www.worldometers.info/coronavirus/?fbclid=IwAR35ZFiRZJ8tyBCwazX2N-k7yJjZOLDQiZSA_MsJAfdK74s8f2a_Dgx4iVk.