Pregnant animals have increased metabolic demand that continues beyond birth and into lactation. The energy required to sustain pregnancy and lactation has to come from somewhere and may require an animal to divert energy spent on its immune system to other parts of its physiology.
Two broad categories of such disease are metabolic and infectious/contagious. Metabolic diseases include such conditions as hypocalcaemia (milk fever), hypomagnesaemia (staggers), pregnancy toxaemia (sleepy sickness), and ketosis. These are generally prevented through diet or supplementation and can be treated with over-the-counter remedies.
Infectious and contagious diseases can include conditions like abortion storms caused by Toxoplasma or Campylobacter, and outbreaks of diarrhoea caused by Salmonella or Rotavirus. These conditions can be rapid in their spread and severe in their effects. All of the diseases mentioned here can be prevented by effective vaccination. It is highly recommended that abortions or clusters of sick or dead animals are investigated by a veterinarian as early as possible in order to identify the cause and prevent any outbreak from worsening.
Remember also to wash your hands and keep your equipment clean, as bacteria like Salmonella and Campylobacter can cause disease in humans as well.