Mould infections are rare, but invasive infections can cause serious or life-threatening illness. The greatest risk is among people whose immune systems are weakened by illness or treatment. Most healthy people breathe in mould spores without developing an infection.
Mould exposure and mould infection are different. Exposure in a damp or mould-damaged building can cause or worsen respiratory irritation, allergies and asthma. An invasive infection occurs when a fungus grows in tissues, blood vessels or organs.
A CDC surveillance report published in 2026 illustrates the severity of invasive mould disease. Its findings do not mean that ordinary household mould poses the same risk to everyone exposed to it.
What is a mould infection?
A mould infection occurs when the mould fungus causing the disease enters the body and begins to grow, for example, in the lungs, sinuses, skin or other tissues. Most serious infections originate from inhaled spores. Less commonly, the fungus can enter the body through a wound, damaged skin or a contaminated medical device.
Mould-related illnesses include three broad groups. Allergic reactions are illnesses, but they are not infections:
- Allergic reactions, in which the immune system reacts to inhaled mold spores.
- Local or chronic infections that can develop, for example, in previously damaged lungs or sinuses.
- Invasive mold infections, in which the fungus invades tissues or blood vessels and can spread to other organs.
Invasive infections are the most serious of these. The CDC defines them as infections that affect blood vessels, deep tissues, or internal organs. They occur especially in people whose immune system is significantly weakened.
Mold exposure and mold infection are not the same thing
| Mold exposure | Mold fungus infection |
|---|---|
| Mould spores or microbial impurities are inhaled from the environment. | Mold fungus grows in the body’s tissue or organ. |
| May be related to a moisture-damaged building. | Often associated with a weakened immune system or a serious illness. |
| May cause irritation or allergy symptoms and worsen asthma. | May cause fever, cough, shortness of breath and a serious generalized infection. |
| Does not mean that a person has a fungal infection. | Diagnosis requires examinations carried out in health care. |
| The condition of the building and the source of exposure can be investigated. | Treatment is always part of health care. |
Therefore, the discovery of mould or microbes in a building does not indicate that the user of the building has a mould fungal infection. Similarly, the symptoms cannot be used to conclude that there is moisture or mould damage in the building. The person’s state of health and the condition of the building are examined using different methods.
What did the CDC study find?
Surveillance at four Atlanta-area hospitals and their outpatient clinics identified 449 invasive mould disease cases during 2020–2024: 89 proven cases, 142 probable cases and 218 cases meeting the study’s surveillance definition.
In-hospital mortality was 30%. Among the 2020–2023 patients with available 90-day follow-up, all-cause mortality was 45%.
The figure refers to mortality calculated from all causes. In other words, the study did not show that mould infection alone had caused all deaths. The patients were often already seriously ill, and many had an illness or treatment that weakened the immune system.
In the study, 65% of patients had at least one recognised risk factor for invasive mould infection. No predefined risk factor was identified in the remaining 35%. This does not mean that those patients were otherwise healthy: they may have had other conditions or factors that increased their risk of infection.
Current or recent COVID-19 was associated with more severe illness. ICU admission in the two weeks before specimen collection occurred in 66% of those patients, compared with 39% of other patients. Ninety-day all-cause mortality was also higher in the COVID-19 group. See the original CDC report for the study methods and limitations.
Which moulds can cause infections?
Serious infections can be caused by several mold fungi that are present in the environment. In the CDC study, fungi of the genus Aspergillus were the most common, which were found in 71 percent of cases.
The most common single identified species was Aspergillus fumigatus. It was found in 21 per cent of all cases. Aspergillus fungi are commonly found in soil, decaying plant material, compost, dust, and outdoor and indoor air.
Other groups of mould fungi that cause invasive infections include:
- Fungi of the Mucorales group that can cause mucormycosis
- Fungi of the genus Fusarium
- Fungi of the genera Scedosporium and Lomentospora
- some dark-pigmented mold fungi
The species of the mould fungus cannot be identified by the colour or appearance of the growth. It is also not possible to determine whether a mould fungus found in a building causes symptoms or an infection in a particular person.
What is aspergillosis?
Aspergillosis is a collective term for diseases caused by Aspergillus fungi. There are several different forms of aspergillosis, with varying degrees of severity and symptoms.
Allergic aspergillosis
In allergic aspergillosis, the body reacts strongly to the spores of the Aspergillus fungus. It can occur especially in people with asthma or cystic fibrosis. Symptoms may include wheezing, coughing, and worsening asthma symptoms.
Chronic pulmonary aspergillosis
Chronic aspergillosis can develop in a person who already has a disease that damages the structure of the lungs. The infection usually progresses more slowly than invasive aspergillosis and can cause, for example, prolonged coughing, fatigue, weight loss or coughing up blood.
Invasive aspergillosis
Invasive aspergillosis is a serious infection in which a fungus grows into lung tissue and can spread through blood vessels to other parts of the body. It occurs especially in severely immunocompromised or critically ill patients.
Who belongs to the risk groups for invasive mold infection?
The risk of a serious mould infection is greatest when the human immune system is unable to fight off inhaled spores normally.
Risk groups may include, for example:
- people with blood cancers, such as leukaemia;
- those who have received a stem cell transplant or organ transplant
- people receiving chemotherapy or other treatment that severely weakens the immune system
- people receiving long-term or high-dose corticosteroid treatment
- people with severe neutropenia, meaning a very low level of infection-fighting neutrophils
- those in intensive care and critically ill
- people who have had severe influenza or COVID-19
- people with severe or advanced lung disease
Old age alone does not automatically mean severe immunodeficiency, but the prevalence of diseases and treatments that affect the immune system increases with age.
What symptoms can a mold infection cause?
Infections caused by mold fungi can manifest themselves in different ways depending on the type, location, causative agent and health of the person. Possible symptoms of invasive aspergillosis of the lungs include:
- fever that does not respond to standard antibiotic therapy
- cough
- shortness of breath
- chest pain
- coughing up blood
- severe fatigue and deterioration of general condition
These symptoms also occur in more common respiratory illnesses. They cannot diagnose a mould infection at home.
If a serious infection spreads outside the lungs, the symptoms may be related to the skin, eyes, central nervous system or other organs, for example. In this case, it is a condition that requires urgent hospital care.
Can mold in the home cause a life-threatening infection?
Environmental spores can be inhaled at home or elsewhere. Most healthy people do not develop infection after ordinary exposure.
A damp building does not, by itself, establish that its occupants will develop invasive mould disease. Exposure-related irritation and allergic symptoms differ from a deep tissue infection.
However, mould exposure in the environment of a person in a risk group should be treated with care. For example, extensive water damage, floods or repair work can release a lot of dust and spores into the air. The treating healthcare unit can provide individual instructions on what kind of environments or renovation situations a severely immunocompromised person should avoid.
How is a mold infection diagnosed?
A mould infection cannot be diagnosed with an indoor air test, a mould test done at home or on the basis of symptoms alone. Diagnosis takes place in health care and may require combining several test results.
Examinations may include, for example:
- X-ray or computed tomography examination of the lungs
- blood tests and testing for fungal antigens
- culture of respiratory or tissue samples
- PCR tests to identify the fungal genome
- Sample taken in bronchoscopy
- if necessary, a tissue sample, i.e. a biopsy
Any indoor air problems in the building and the person’s illness are investigated separately. A finding in a building is not a medical diagnosis, and a negative building sample does not rule out the person’s illness.
How are mold infections treated?
Treatment is determined by the cause, location, severity of the infection and the patient’s state of health. Invasive mould infections are treated with antifungal drugs, and treatment is often started in a hospital.
In some situations, surgical treatment is additionally required to remove the damaged tissue. At the same time, the aim is to reduce the factors that weaken the immune system as far as possible. The treatment can be long, and the effectiveness of one antifungal treatment is not the same against all mould fungi.
Mould or indoor air tests used at home are not suitable for selecting or monitoring the treatment of an infection.
When should the indoor air of a building be examined?
An examination of the indoor air or surfaces of a building can be considered if the following are observed in the space, for example:
- visible mould growth or repeated discolouration
- musty or cellar-like smell
- traces of moisture, leaks or peeling of coatings
- previous water damage where complete drying has not been confirmed
- recurring symptoms associated with a particular space that improve elsewhere
Preliminary indoor air and surface tests can provide additional information about the sample to be examined according to the manufacturer’s intended use. However, they do not determine the location or extent of the damage and do not replace a structural survey.
Compare indoor air and mould testing options. For medical background, read CDC’s information on invasive mould infections and NHS guidance on aspergillosis.
For further reading on symptoms, see CDC’s symptoms of aspergillosis.
Can floods and climate change increase the risk of mold infections?
Floods, storms and other events causing extensive water damage can increase moisture damage and mould growth in buildings. Removing damaged materials or disturbing dried mould growth can release unusually large amounts of dust and mould spores into the air.
The CDC has estimated that the increasing frequency of extreme weather phenomena related to climate change may increase the importance of mold exposure in the future. At the same time, the ageing of the population and the increase in treatments that affect the immune system may increase the number of people susceptible to serious infections.
However, a study conducted by the CDC in Atlanta hospitals did not determine where individual patients had been exposed. Based on this, infections cannot be directly linked to patients’ homes, workplaces or hospital buildings.
When should you seek treatment for symptoms?
Most mild respiratory symptoms are not caused by an invasive mould infection. However, it is advisable to contact health care if respiratory symptoms are prolonged, become more severe or are accompanied by a clear deterioration in general condition.
An urgent assessment is especially necessary if a person in a risk group has:
- prolonged or high fever
- worsening shortness of breath
- chest pain
- coughing up blood
- confusion or rapidly deteriorating general well-being
For sudden severe breathing difficulty or another emergency, call the local emergency number; in Finland and the EU, call 112.
Summary of infections caused by mold fungus
Mould-related illnesses include allergic reactions, chronic lung conditions and rare invasive infections. Allergic reactions are not infections. Serious invasive infections mainly affect people with weakened immune systems or those who are critically ill.
Normal mold exposure in a building does not mean that a person has a mold infection. However, it is still worth investigating moisture or mould damage in your home and workplace, as they can deteriorate indoor air quality, be associated with irritation and allergy symptoms, and damage the building.
An indoor air test can support a preliminary assessment of the condition of a building, but it cannot be used to diagnose a person’s illness. Suspected mould infections are always examined and treated in health care.
