When we went to South America, Argentina, Uruguay and Brazil we were told to get a Yellow Fever vaccination, but at the time, for some reason none were available. However, we were lucky we entered Argentina first and they didn't require that coming from England and neither did Brazil as that was our second port of call and in any case we 'd been in urban areas, apart from Iguazu which we were told was pretty safe, I did get all manner of repellents which contained an effective element called "deet" in combating bites. When we were in Brazil we didn't go anywhere near The Pantanal or the Amazon where it's advisable to have the vaccination I think.
We have friends who did that cruise up the Amazon to Manaus, I don't think they had the YF injection either, but can't swear to that. Yes apparently once over a certain age it can produce a nasty reaction.