I guess it could be worse.
Sometimes people develop M.E. following an infection, such as glandular fever. For other people, their illness may have been triggered by a significant amount of stress. A combination of an infection and stress is commonly reported. Occasionally, it seems to come “out of the blue”.
Is there any evidence to support this idea that stress is a significant common contributing factor? It's not my experience. I'd change that whole paragraph to
'Frequently ME develops after an infection, such as glandular fever'.
and leave it at that.
The exact mechanisms underlying M.E. are still unknown. Our current understanding is that the nervous system and possibly the immune and endocrine systems of the body are likely to be involved in M.E. These systems are complex and operate at a deep level inside us. For this reason, someone with M.E. may look well on the outside, even if they are having a really bad day.
A deep level inside us?

How deep can it be? Existentially deep?
I've heard this thing about the endocrine system being involved but I haven't seen much evidence of it being a primary cause. Highlighting it seems to allow people to head down 'adrenal exhaustion' paths with stress management being the solution. My son saw a very good endocrinologist and she found no endocrine problems.
I'd change it to
'The mechanisms underlying M.E. are still unknown. There is evidence that the body's ability to produce energy is compromised and the immune system is dysfunctional. Someone with ME can look healthy but feel very unwell.
With M.E., the body’s systems are thought to have become poorly regulated and hypersensitive. The body is in a constant state of “high alert” almost as if a switch has been thrown and the whole system is in a state of emergency. This is thought to be why the smallest of stressors such as noise, light, or physical activity can have a big impact on the symptoms of M.E. Often too much stimulation from things such as noise or light can result in someone with M.E. feeling worse the next day.
This paragraph seems speculative and, yes, as
@adambeyoncelowe said, seems to be CNS sensitisation propaganda. I'd leave the whole paragraph out unless chemical, light or noise sensitivities are significant problems, in which case I'd just state that.
People with M.E. often have varying symptoms from one day to another, or even within the same day. This can make it difficult to decide whether or not to attempt an activity. It can be tempting to try and do more on a better day. However, if someone with M.E. does too much in one day, they are likely to feel worse afterwards and have several days recovering. This can lead to big swings in activity over time, known as “boom and bust”. This boom and bust pattern is unsustainable and over time and leads to a reduction in the level of activity a person can manage.
Boom and bust - we don't need that patient blaming terminology. Sometimes doing too much is unavoidable.
'People with M.E. often have varying symptoms from one day to another, or even within the same day. If someone with M.E. does too much in one day, they are likely to feel worse afterwards for several days. Repeatedly doing too much can result in longer term decreases in health.'