https://www.scientificamerican.com/article/how-a-warming-climate-could-affect-the-spread-of-diseases-similar-to-covid-19/

Scientists have long known that the rise in average global temperatures is expanding the geographical presence of vector-borne diseases such as malaria and dengue fever, because the animals that transmit them are adapting to more widespread areas. The link between respiratory illnesses, including influenza and COVID-19, and a warming planet is less clear. But some scientists are concerned that climate change could alter the relationship between our body’s defenses and such pathogens. These modifications could include the adaptation of microbes to a warming world, changes in how viruses and bacteria interact with their animal hosts, and a weakened human immune response.

The immune system is our natural defense against harmful substances. When a respiratory pathogen—such as the new SARS-CoV-2 virus that causes COVID-19—enters the body through the airways, it damages cells by taking over their machinery and making more copies of itself. The injured cells release signaling proteins called cytokines that communicate with other parts of the body to activate an immune response against the foreign invaders.