When a few weeds appear on your front lawn, you can easily pick them off one by one. But if they start taking over the yard, the picking becomes laborious, and you may need to turn to a chemical weed-killer to hold the invaders in check. After several applications of the herbicide, however, the weeds could become resistant, forcing you to use an even more powerful solution. Meanwhile, the survival of your lawn hangs in the balance. Like weeds in a lawn, pathogenic fungi and yeasts (single-celled fungi) can invade and overtake our bodies. In people with healthy immune systems, cells called macrophages and neutrophils engulf these pathogens, nipping them in the bud. But when the immune system is weakened by disease or drugs, fungi ? much like weeds in your garden ? can grow unchecked.