Disease and recovery in wildlife

Wild animals, like domestic animals and humans, are constantly exposed to disease-causing organisms. Bacteria, viruses, parasites, fungi, and prions infect wildlife around the world, yet their effects often remain unseen. While some pathogenic infections cause obvious sickness and death, others may cause few signs, affect only some individuals, or even persist silently within apparently healthy animals. Understanding how diseases affect wild species—and how these animals fight, conceal, or recover from infection—reveals both the resilience of wildlife and the complex relationships between hosts and pathogens.

Pathogens have the potential to infect various animal hosts, but different host species may not always present sickness in the same way. For example, avian influenza, or bird flu, is a virus which can cause both asymptomatic and symptomatic infections in wild birds. An animal may get sick with bird flu when exposed to other sick animals or virus-contaminated environments. After exposure, the resulting sickness can look strikingly different in each host. In some wild geese and ducks, a bird may be infected but show no obvious signs of illness, an example of asymptomatic infection. However, other birds like pelicans, cormorants, and sanderlings are more likely to show clear signs of disease including trouble breathing, flying, or maintaining balance, as well as die from the infection. These bird species experience symptomatic infection, meaning the virus causes clear signs of illness. While each of these species are infected with the same virus, the resulting disease looks very different from animal to animal, reflecting variation in host susceptibility and infection pathogenicity. 

The challenge of latent infections 

For some diseases, a successful immune response leads to recovery when the pathogen is eliminated from within the host’s body. In other cases, the pathogen is not fully cleared but instead enters a dormant or latent state, remaining in the body without causing active disease. This condition, known as latent infection, can persist for long periods as the immune system keeps the pathogen in check. Many viruses undergo latency, during which viral replication is minimal or absent.

Recovery with latent infection can happen in some warm-blooded animals after infection by the parasite Toxoplasma gondii which causes the disease toxoplasmosis. In response to this infection, the host’s immune system limits parasite replication which restricts the spread of Toxoplasma gondii through the body. In turn, the parasite transitions into a dormant stage, forming tissue cysts of bradyzoites that can persist for the lifetime of the host. This encysted state represents an evolutionary adaptation that benefits both parasite and host: the host avoids severe disease, while the parasite survives undetected and retains the potential for transmission to new hosts. As a result, many animals can recover from the acute phase of toxoplasmosis while remaining chronically infected.