>4) There is no known microbe which is beneficial for some humans, and harmful
>for others (micro-organisms responsibles for diseases like TB or measles
have no known beneficial effect).

Well, you could consider that it is beneficial to be infected with measles
when still a young child, because it then gives you immunity for when you
become adult (if you do ;), as adult onset measles has far greater risk.

>5) The only known beneficial micro-organisms are those which live in
>the digestive tract (correct me if I am wrong).

I have read that similar organisms are in breast milk.

> -some animals are predators (harmful)
> -some animals are preys (beneficial)
> -some animals are neutral (not predators, not preys)
> -some animals are not predators nor preys, but since they compete against
>others, they help control the growth of various populations
> -obviously, the weakest animals are the most likely to become victims of
>predators.

Predators may be harmful to some animals of a species, but they are usually
beneficial to the species as a whole: ie if the numbers of a specy (?) are
allowed to multiply unchecked then ultimately the food supply (eg grass)
would be decimated to the point where none would survive.  [ This may be the
reason why lemmings commit wholesale suicide when they are in big numbers].

Regards,
Lynton