Explanation:
During the yellow fever epidemic in Philadelphia in the late 18th century, people who left the city indeed appeared to have milder cases of yellow fever or avoided the infection altogether. Several factors contributed to this phenomenon:
1. **Reduced Exposure to Infected Mosquitoes:** Yellow fever is primarily transmitted through the bite of infected Aedes mosquitoes. By leaving the city, individuals reduced their exposure to these mosquitoes, decreasing the likelihood of being bitten and infected.
2. **Avoidance of Urban Crowding:** Cities, especially during outbreaks, can become overcrowded and congested, providing an ideal environment for disease transmission. People leaving the city likely avoided the crowded conditions that facilitate the spread of the disease.
3. **Improved Sanitation and Living Conditions:** In some cases, individuals who left the city sought refuge in areas with better sanitation and healthier living conditions. Clean and hygienic environments reduce the likelihood of disease transmission and can contribute to better health outcomes.
4. **Access to Fresh Air and Open Spaces:** Leaving the city granted individuals access to fresh air and open spaces, which can have positive effects on overall health and immune function. These factors might have contributed to a milder manifestation of the illness or helped in avoiding infection altogether.
Overall, leaving the city during the yellow fever epidemic in Philadelphia provided individuals with a reduced risk of exposure to the disease vectors (mosquitoes), improved living conditions, and a healthier environment, all of which contributed to milder cases or avoiding the infection.
