Deadly fungal infections pose a hidden risk to US insurers as the city of Joplin, Missouri continues to count the cost of a tornado that ripped through the region last month.

Eqecat estimates of insured losses between $US1 billion ($950 million) and $US3 billion ($2.85 billion) remain unchanged, but concerns are now growing over risks associated with potentially fatal infections that result from the tornadoes.

Media reports last week said nine people had been admitted to hospital and four had died from fungal infections.

Mycologist Sapphire McMullan-Fisher says such infections at times of natural disaster are not uncommon.

She says toxic growths tend to be attracted to and grow in wet, humid and warm conditions and can prosper under the common household sink.

“When you have the damage and dispersal of houses as you get in tornado conditions, and it’s not cleaned up quickly enough, mould can grow and it gets into the air,” Dr McMullan-Fisher told insuranceNEWS.com.au.

She says the other danger is that the human immune system doesn’t always pick up fungus as a threat, rather seeing it as a plant species. People with immunity problems are at greater risk of adverse reactions.

The US Centres for Disease Control and Prevention is now investigating the matter to identify any risks association with contracting fungal infections.