Nutritional deficiency and Pesticides can be other contributing reasons, so it's essential that sufficient knowledge is attained to be able to differentiate which is what and how to deal with it.
The bee's lifecycle can be divided into two phases which is the larval phase and the adult phase and each phase is susceptible to its own set diseases
Bees Diseases :
(1) European Foul Brood :
Is caused by : bacterium melissococcus plutonius
the bacteria feeds on food that's in the gut of the larval starving it to death.
The disease can be detected by unnatural position of the larval and color changes, which is from the pearly white (this indicates health) to a creamy or brown color.
(2) American Foul Brood :
Is caused by : spore forming bacterium larvae, whereby the brood food is contaminated.
Once contaminated they develop into a bacterium that penetrates the gut wall then multiply within the tissues of the larval causing the larvae to die from blood poisoning.
It affects the bee larvae only but highly contagious and deadly to the brood.
(3) Nosema :
Is caused by Nosima Apis that is a spore forming protozoa, it able to multiply in the bees stomach, causing digestion of pollen to be impaired, the end result is dead bees.
Symptoms :
The most notable symptom is dysentery. This appears as yellow stripes on the outside of the hive and in severe cases, inside the hive. Bees may be unable to fly ("crawling") due to disjointed wings.
(4) Chalk Brood :
Is caused by : the fungus Ascosphaera apis.
the fungus Ascosphaera apis infests the gut of the larva.
The fungus will compete with the larva for food, ultimately causing it to starve.
The fungus will then go on to consume the rest of the larva's body, causing it to appear white and 'chalky'.
This kind of disease is common with expanding colonies and it's still unclear what causes but it can be attributed to high Carbon dioxide levels in the nest if it's not properly ventilated or pollen deficiencies.
Chalkbrood is most commonly visible during wet springs.
Hives with Chalkbrood can generally be recovered by increasing the ventilation through the hive.