$(a)$ Mention the harmful effects of infectious diseases on the health of cattle.
$(b)$ How can these diseases be controlled?

(N/A) Infectious diseases in cattle lead to a significant reduction in milk production,loss of body weight,and in severe cases,can lead to the death of the animal.
$(b)$ These diseases can be controlled by:
$1$. Proper vaccination of cattle against common diseases.
$2$. Maintaining hygiene and cleanliness in the cattle shed.
$3$. Providing a balanced diet and clean drinking water.
$4$. Isolating infected animals to prevent the spread of infection.