There is no organism known to be completely immune to all diseases. Every living organism, from the smallest bacteria to the largest animal, is susceptible to some form of disease.

Here's why:

* Evolutionary Arms Race: Diseases constantly evolve and adapt to their hosts. If an organism were completely immune, the disease wouldn't be able to survive, leading to its extinction. This means there's always a chance for new diseases to emerge that can affect even previously immune organisms.

* Diversity of Pathogens: There are countless types of pathogens (viruses, bacteria, fungi, parasites) that can cause diseases. An organism might be resistant to one type, but vulnerable to another.

* Individual Variation: Even within the same species, individuals can have different levels of immunity due to genetic factors, lifestyle, and environmental influences.

While some organisms have strong immune systems and are resistant to many diseases, complete immunity is impossible.