Is fever (temperature) below 106° F harmful?

Question

Parents and pediatricians rush to treat low grade fever with acetaminophen and other powerful pain killers. Medical researchers refer to the over zealous treatment of non hazardous fever as fever phobia.

In his book, How to Raise a Healthy Child in Spite of Your Doctor, Robert S. Mendelsohn, MD writes that fever under 106 F is not dangerous in children 3+ months of age:

Most fevers are caused by viral and bacterial infections that the body's own defense mechanisms will overcome without medical help..

The common cold and influenza are the most common sources of elevated temperatures.. they can generate fevers that range all the way up to 105 degrees, but even at that level they are not a legitimate cause for alarm..

Untreated fevers caused by viral and bacterial infections do not rise inexorably and will not exceed 105 degrees.. (I suppose this includes anything below 106 as he explicitly states in next paragraph)

Only in the case of heatstroke, poisoning, or other externally caused fevers is this bodily mechanism over-whelmed and inoperative. It is in those cases that temperatures reach and exceed 106 degrees..

Is there any evidence that suggests that fever (from viral or bacterial origin) below 106 F has any long term negative effects in otherwise healthy children/toddlers/adults?

Similarly, in a report entitled Fever and Antipyretic Use in Children the authors write that:

Hyperthermia should be addressed promptly, because at temperatures above 41°C (105.8 F) to 42°C (107.6 F), adverse physiologic effects begin to occur. Studies of health care workers, including physicians, have revealed that most believe that the risk of heat-related adverse outcomes is increased with temperatures above 40°C (104°F), although this belief is not justified.

Note:

To be clear, it doesn't matter if you think fever is pleasant or not. Or if the benefit of reducing the pain caused by fever outweighs the benefit that fever provides in helping fight infection.

My question is simply: Are there any dangers associated with fever within the above mentioned range?

Answer 1

Your source claims that

The common cold and influenza are the most common sources of elevated temperatures.. they can generate fevers that range all the way up to 105 degrees, but even at that level they are not a legitimate cause for alarm..

According to the CDC:

On December 9, 2016, CDC posted estimates of seasonal flu deaths from more recent seasons in the United States. CDC estimates that from 2010-2011 to 2013-2014, influenza-associated deaths in the United States ranged from a low of 12,000 (during 2011-2012) to a high of 56,000 (during 2012-2013). Death certificate data and weekly influenza virus surveillance information was used to estimate how many flu-related deaths occurred among people whose underlying cause of death on their death certificate included respiratory or circulatory causes.

In addition to the number of deaths, the flu causes hundreds of thousands of hospitalizations each year. Is that a legitimate cause of concern? Considering that children under 5 are a high risk group for serious complications, and that high fever is one of the symptoms that physicians consider in deciding if the patient has the flu or more benign viral infections, I think high fever is indeed a red flag that needs to be taken seriously.

Another part of this claim appears to be that regardless of the underlying condition, fever up to 105 degrees should be left untreated:

Untreated fevers caused by viral and bacterial infections do not rise inexorably and will not exceed 105 degrees..

This is more difficult to answer. There is certainly a body of evidence that suggest that parents tend to overstate the risks of fever and over-treat it. There have also been some studies that suggest that fever might have positive effects on the underlying infection.

While leaving fever untreated won't have any long term negative effects on the patient, and might indeed have some positive physiological effects, we have to consider other factors, such as the well being of the patient. The modern Hippocratic oath that is widely used in US medical schools states:

I will remember that I do not treat a fever chart, a cancerous growth, but a sick human being, whose illness may affect the person's family and economic stability. My responsibility includes these related problems, if I am to care adequately for the sick.

Considering all of that, I think the answer is:

Leaving fever untreated will not cause long term negative effects on the health of the patient, but high fever is a red flag that could indicate serious illness and as such should be taken seriously. A doctor can decide to treat the fever in order to make the patient or their family comfortable and there is nothing wrong with that.