When you are going through treatment for blood cancer, at some point you will hear your care team say, "Your white cell count has dropped quite a bit this week." That happens because chemotherapy attacks fast-growing cells, and the bone marrow gets caught up in that process too. The period when neutrophils - the cells that fight off bacteria - fall is the one to be most careful about. During this time, even a small infection that you would normally shrug off can turn into something serious, so every little thing you eat starts to feel like it matters.
The first thing to pay attention to is food safety, in other words, hygiene. A body with weakened immunity has a hard time clearing out the germs found in raw foods on its own. Things like raw fish, raw beef, undercooked meat, unpasteurized raw milk or cheese, and freshly squeezed juices are best kept at arm's length for now - it's simply less worrying that way. Wash vegetables and fruit thoroughly and peel them, and heating or cooking them once gives you extra peace of mind. Stick to food that was made recently, and even if it feels wasteful, it's better to throw out leftovers without a second thought.
That said, if you end up cutting out so much that you simply eat less overall, that becomes the bigger problem. There will be plenty of days when you have no appetite and your mouth is too sore to swallow, but if protein runs short at times like these, even the recovery of your fallen blood counts slows down. It helps to eat small amounts often of foods that are easy to digest yet rich in protein - well-cooked eggs, soft simmered tofu, thoroughly boiled chicken breast, white fish. If you can't manage much at one sitting, it's perfectly fine to split the day into several smaller meals.
When your mouth hurts or you feel nauseous, gentle, soft foods go down more easily - rice porridge, thin gruel, mashed potato, warmed soy milk. Food that is too hot, spicy or salty, and sour fruit juices, only make a sore mouth sting more, so let things cool to lukewarm before eating. It's also good to sip water often, a little at a time. When you actually set out to do it, drinking water is surprisingly easy to forget, but the small habit of keeping a cup somewhere in plain sight can make a bigger difference than you'd expect.
You may feel tempted to add health supplements, "immune-boosting" juices, or fermented drinks on your own, but please be sure to talk it over with your care team before starting any of them. Products containing live cultures and some supplements can actually be a burden during a time when your immunity is low. And if you run a fever of 38 degrees Celsius or higher, get the chills, or develop sudden pain, the priority is to contact the hospital right away rather than worrying about your diet.
What is written here is only general reference. Because each person's treatment stage and physical condition is different, please be sure to work out your actual diet together with your care team and a dietitian.