First off, you never medicate unless it's absolutely necessary. The reason being is that if you offer or give medications when the chickens are not sick, their bodies will only build resistance to the medication. Save the medications for when you will need them, if ever. With good flock management and a predator proof run or free ranging area, that day might never come.
Second, the theory of most birds dying at ages 3 to 6 months is just not true. There are many people on here who have never lost a chicken, let alone at that age. Again, if you are managing your flock properly, you shouldn't have any problems.
Third, there are immunizations available and you can get them at places like
www.jefferslivestock.com . There are many who never immunize and never have problems, then there are some who do. If you plan to show your birds, then of course, immunizations would be an absolute must. If not, then the most common illness' to vaccinate against would be Marek's, which can only be done in day old chicks to be effective, and Infectious Bronchitis (IB)/Newcastle Disease. Both of those can be done by yourself, or most hatcheries (if that's where you choose to get your chicks from) offer those vaccinations at a very nominal price.
I mentioned good flock management several times. Here is a link to a thread started by speckledhen on good flock management. It's a great thread! You'll have to check it out.
https://www.backyardchickens.com/forum/viewtopic.php?id=129278
Happy chicken keeping!