Although being a wolfman I am an anomaly to this, humans are losing their body hair as an evolutionary process.
And what am I then? :P
Saying that humans have the best immune system is just a bunch of nonsense if you ask me.
Sure our immune system is able to cope with a wide range of various pathogens, but the development of medicine is the main cause why our immune systems work so well.
Just a few hundred years ago people were dying of common cold and various secondary infections that are easily treatable with antibiotics. Not to mention the outbreaks of plague.
And the fact is that our hygienic standards are much higher, so we keep the amount of various microorganism relatively low in our environment. Animals don't have that luxury.
We also control our environment how we please. That's a huge factor to consider.
Guess one: pathogens require another thing to exist which is why they are not (until further classification) technically living.
Not necessarily.
Viruses, for example are always classified as something between living (they carry genetic material, reproduce, evolve) and non-living (they lack cell structures). Here you may be right.
But looking at bacteria, like E.coli for example, you're wrong. Essentially a beneficent inhabitant of our intestines (produces vitamin K2, prevents other bacteria from settling in), but some variations can cause serious food poisoning. There are many other similar examples, but this one is the most common. Obligate pathogens are not the only kind of pathogens, as you know.
Bacillus anthracis, which causes anthrax usually inhabits the soil (and in relatively huge quantities, if I remember correctly).
Parasites should also be included here... Even though they are not microorganisms they still make your immune system go wild. Consider Trichinella spiralis and it's life cycle as an example... The main reason why it's eggs remain incrusted in human muscles is because our immune system tries to neutralize the threat (unsuccessfully, though).
Guess two: pathogens 'like' energy. Energy in this sense is from heat from the host which is why humans are a better species for the pathogens to evolve to infect than, say, reptiles or fish.
Not entirely true.
I've had an aquarium for a number of years and I can tell you that fishes do get sick a lot.
It's just that fishes are usually more prone to fungal infections and parasites. Bacterial infections like fin rot are still present, though.
I don't know much about the reptiles unfortunately.
Guess three: pathogens have been trying to evolve to infect humans (being the ideal host for pathogens) for so long that they have 'forgotten' to evolve to infect anything else which is why there are so many more diseases that affect humans than animals. That is also a guess.
Incorrect. Consult this wonderful chart:
(https://lh5.googleusercontent.com/NRThyNLRuoxWtyv035-Bw3fl4c3Y5I9ay2UGX8LSBE2SJosHLkxGhQpwZDpEbiuE1TPZU0kwC5ydMaanv3OZWLded_bm-Wr2w7Kl0yZOQL8MU0zh5C0U42_ESg)
You'll see that birds carry the most types of influenza. Which is completely understandable when you consider the way chicken are breeded now-days. Huge populations in crowded spaces, artificial lights, non-balanced diet and so on... All of that has impact on their immune systems and makes the job easier for pathogens.
And then there are pigs.
Also one of the most common farm animals and the most dangerous vector (judging from the human perspective) for various strands of the disease.
Probably due to similarity of our immune systems.
So anyway I'm not too impressed with how little I know on the subject or on how vague and possibly incoherent my thoughts are. :(
Same here. I'm feeling a bit stupid being a third-year student of biology...