Penguins are susceptible to a range of infectious and parasitic diseases. Birds can be more susceptible to disease if they are stressed (environmentally or nutritionally) and their immune system is suppressed.
Avian diphtheria is a disease which commonly affects young chicks. Bacterial plaques form in the mouth and can be inhaled, causing aspiration pneumonia, or result in a blockage which prevents feeding and causes dehydration. Chicks which survive have reduced growth rates and often fledge in poor condition.
Avian malaria is caused by blood parasites (Plasmodium sp.) which are transmitted by mosquitoes. In the season 2018/19 there were 29 confirmed hoiho deaths from avian malaria which affected adults as well as chicks. It is likely that significant rain events and increased temperatures led to favourable breeding opportunities for mosquitoes, thus increasing their numbers.
Disease has the potential to become a bigger issue in the future as the climate warms, and as human disturbance and stress on the penguin population increases.