Do humans have a weaker than normal immune system, or just we modern western humans aren’t accustomed to fight of infections because we haven’t been exposed to them? I tried to research about it, but no conclusive results. I haven’t found anywhere that human immune systems are objectively less able than those of other studied animals. What I found was that hunters and gatherers were extremely healthy, that epidemics started to rage when humans adopted agriculture and starting having lower quality food, more children and lived in much higher densities, and that modern human susceptibility to disease is explained by the hygien hypothesis – too clean living conditions. Not all experts agree with the latter though, pointing out that traditional people payed resistance to disease with increased child mortality. I want to have a satisfactory and simple answer, because the topic is confusing. Thank you all very much.

This is a good place to start:

http://en.wikipedia.org/wiki/Hygiene_hypothesis

And no I wouldn't say that human immune systems are weaker than animal ones. They have evolved to deal with different organisms because of the varying environments in which different animals live.

As Reetika says, the immune systems of animals are adaptations to their environment and behaviour. In terms of immune system ‘strength’ the antimicrobial activities of serum from American alligators (often living in territory teeming with bacteria and other microbes) are very impressive (see - http://www.ncbi.nlm.nih.gov/pubmed/16298430 and commentary at http://news.nationalgeographic.com/news … blood.html which highlights the apparent broader range antimicrobial activity of alligator versus human serum). Sharks and bats also have very effective innate immune systems.

And domestic animals such as cows tend to have very weak immune systems, which is why dairy farmers have to deal with unexpected sudden deaths in their herds all the time. This is because the breeds we have today have been very selectively bred for milk-yield above all other considerations.