What animal has the strongest immune system?

Question

I'm wondering what animal has the strongest immune system. It can be defined as the most evolved immune system or the immune system that can eliminate or tolerate most number of (different) viruses/bacteria/diseases/pathogens/poison (some of which can kill most or many other animals or make them really sick).

I've done some research and found some animals that can be the answer but I couldn't be sure which one. There might be smaller animals that I've missed. There might be studies with more clear results that I've missed. Here are the animals I've found:

• Shark
• Ostrich
• Bat
• Alligator
• Vulture

Here are some relevant quotes from the articles I've found:

• Shark:

The shark's "antiviral defenses have been extraordinary," Zasloff said. "It has adapted a very remarkable immune system and stayed with it." In the study, squalamine thwarted infection of the dengue fever virus in human blood vessel cells and of hepatitis B and D in human liver cells—and with little harm to sharks. _{https://www.nationalgeographic.com/culture/article/110919-sharks-drugs-medicine-viruses-science-health-squalamine}

• Ostrich:

It is said that ostriches have the strongest immune system of any animal in the world. Because of this, they have shown great promise in preventative healthcare in humans and the beauty industry.

Ostriches strong immune systems make them highly resistant to viral infections. Japanese researchers have discovered how to mass produce natural ostrich antibodies that are effective against influenza, avian flu, MERS, Zika and even the Ebola virus. “The antibodies are resistant to higher pH levels and can remain active even when exposed to heat of up to 100 degrees Celsius.” _{https://pharmaboardroom.com/articles/ostrich-antibodies-a-health-and-beauty-elixir/}

• Bat:

The mammals can carry many viruses with the potential to cause serious diseases in people, including rabies, Ebola, Nipah, severe acute respiratory syndrome, or SARS, and others. Bats rarely get sick from those viruses. Why these pathogens tend to be so dangerous when they infect other animals has been a mystery.

Previous work suggests that a bat’s immune system is especially adapted to tolerate viruses, thanks in part to its ability to limit inflammation. Now a study using cells grown in a lab hints that to counter a bat’s immune defenses, these viruses have gotten good at spreading rapidly from cell to cell. _{https://www.sciencenews.org/article/bats-immune-system-viruses-ebola-marburg-people}

• Alligator:

American alligators have an enviable innate immune system, the “primitive” first line of defence that is shared by all vertebrates. In 2008, chemists in Louisiana found that blood serum taken from the reptiles destroyed 23 strains of bacteria and depleted reserves of the HIV virus. _{https://www.newscientist.com/article/dn27059-germ-killing-molecules-identified-in-alligator-blood/#:~:text=American%20alligators%20have%20an%20enviable,reserves%20of%20the%20HIV%20virus}

• Vulture:

Vultures' faces and large intestines are covered with bacteria that is toxic to most other creatures, but these birds of prey have evolved a strong gut that helps them not get sick from feasting on rotting flesh, according to a new study.

"On one hand, vultures have developed an extremely tough digestive system, which simply acts to destroy the majority of the dangerous bacteria they ingest," Roggenbuck said. "On the other hand, vultures also appear to have developed a tolerance toward some of the deadly bacteria — species that would kill other animals actively seem to flourish in the vulture lower intestine." _{https://www.livescience.com/48899-vultures-bacteria-microbiome.html}

Answer 1

Though not strictly pathogenic in nature, Opossums (Didelphis virginiana) are well known to be able to recover from broad range of deadly toxins/venoms due to -- at least in part -- lethal toxin-neutralizing factor (LTNF).

According to Lipps (1999), who tried putting LTNF into mice:

Death due to intraperitoneal (IP) injection of a predetermined lethal dose of venom from major families of snakes, for instance Crotalidae, Elapidae, Viperidae and Hydrophiidae, is prevented in mice by subsequent IP inoculation of LTNF. Furthermore, LTNF neutralizes the lethality of scorpion and bee venoms and toxins from various animals, plants and bacteria. Thus, natural LTNF from opossum serum has potential as a universal therapy for envenomation caused by animals, plants and bacteria.

Also, because of their low body temperature, opossums rarely if ever get rabies unlike all other mammal species.

Opossums have also been known to eat a wide variety of foodstuffs including carrion, bones, and even garbage without getting sick -- suggesting they have strong immune systems associated with their gut.