Why do some diseases still spread even though vaccines to prevent them exist?

Some diseases continue to spread despite the availability of vaccines due to factors such as vaccine hesitancy, where individuals choose not to vaccinate due to misinformation or personal beliefs. Additionally, vaccines may not provide complete immunity, leading to breakthrough infections, especially in populations with lower vaccination rates. Variants of pathogens can also emerge, potentially evading existing vaccines. Lastly, public health infrastructure challenges can hinder vaccination efforts, allowing diseases to persist.