Homeopaths have recognised for 180 years that while there are disease agents (living organisms that can cause illness), their ability to infect the body depends on an individual’s susceptibility. They claimed that susceptibility could be affected by a person’s underlying state of health or their lifestyle.  Nowadays this is an accepted part of medicine, and the idea that disease agents alone can explain illness is not considered valid:
It is not biological organisms, but human beings, who are ill, and even diseases, like a duodenal ulcer or a cancer, which clearly involve a biological defect, have causes, manifestations and effects which reach far beyond the limits of biology. Therefore, clinical medicine is more than applied biology. Clinicians must also take into account their patients’ experience of pain, suffering, self-respect, aim in life, etc., and they must learn to deal with such non-biological phenomena in a rational way. That is, perhaps, the biggest challenge offered by contemporary medicine. At best, the reduction of non-biological phenomena to biology is futile, and, at worst, it leads to a distorted and unacceptable view of man. 
Susceptibility is increased by stresses (physical, mental or emotional), whether prolonged or sudden, and especially by miasmatic conditions. On the other hand the healthier and less stressed a person is, the less likely they are to become ill.
1. Samuel Hahnemann (trans. William Boericke), Organon of Medicine, 6th edn, manuscript completed 1841, 1st English edn 1921 (Calcutta: Roy Publishing House, repr. edn 1972), § 73 p. 152.
2. Henrik R. Wulff, Stig Andur Pedersen & Raben Rosenberg, Philosophy of medicine: an introduction (Oxford: Blackwell Scientific Publications, 1986), p. 59.