You may need the yellow fever vaccine for your travel destination
The yellow fever vaccine is recommended by the CDC for travel to many countries, particularly in South America and Africa. Many countries--especially in Africa--also require that you get vaccinated against yellow fever at least ten days prior to arriving in country. Yellow fever vaccine is recommended or required in these countries not only to keep you safe but to keep the entire population of the country safe from yellow fever infection. Yellow fever is a terrible, deadly disease and you don't want to get it or be responsible for spreading it to others. Yellow fever is a virus that is spread among its human and non-human primate hosts via mosquitoes.
The yellow fever vaccine is not recommended for the United States
It is not currently a risk in the United States though the country has seen some huge yellow fever outbreaks in its history. Occasionally, the presence of the mosquitoes that spread yellow fever (aedes aegypti) will make the news but there haven't been yellow fever outbreaks in the last 100 years or so.
You don't need a yellow fever shot if you've already had one
If you've had one yellow fever shot in your lifetime, you don't need a booster in most cases. You will likely need to prove you've been vaccinated to enter into countries where it's required. That means keeping track of your yellow international immunizations card. If you've lost your yellow vaccine card you will have to get the yellow fever shot again. You should try reaching out to the clinic where you got the shot originally to see if they still have a record before redoing it because it's pretty expensive. Make sure you don't lose your vaccine card again!
You may want to get a yellow fever waiver instead of the shot if you're in a vulnerable group
If you are over 60 years old or have a compromised immune system, you can get a yellow fever vaccine waiver that should allow you to enter any country where it is required. You can speak to your primary care provider or a travel health specialist to help you make that determination. In particular if you've had your thymus removed or have a thymus disorder, that's important information to share with the provider who is helping you determine if you should get the yellow fever shot.
Infants don't need the yellow fever vaccine
The yellow fever vaccine is not recommended for children under 9 months old.