And do we know how many viruses are bad, good or neutral for their host?

I like to say 1 percent are pathogens, which harm their host, but I think that’s probably a very high estimate. That’s based on early studies with simian [ape and monkey] viruses, which were studied a lot during early days in molecular virology. There were 80-something of these simian viruses they discovered and they were numbered SV1, SV2, etc. Of those, SV40 is the only one that’s had very much study done, and that’s because it turned out to be a pathogen, causing tumors in mice. None of the rest of them had any effect on the host they were tested in. That’s where I get my “about 1 percent.”

You were originally inspired by bacterial viruses, but in your research today, do you have a favorite virus, or one that particularly interests you?

They’re usually found in pretty low numbers, and we don’t know very much about what they do. In some cases, we know they benefit the plant. For example, white clover cryptic virus affects nodulation in legumes. Legumes normally form nodules of bacteria, in their roots, to help them take up nitrogen from the atmosphere. But doing so is costly for the plant. In virus-infected legumes, when there’s enough nitrogen in the soil, then they don’t form nodules, and that’s a benefit to the plant.

We’ve been studying another one called pepper cryptic virus. It’s a very hard thing to prove, but it seems like virus-infected seeds have a lot longer longevity than uninfected ones. After a couple of years, the uninfected seeds don’t germinate, whereas the infected cells last for many years.

Because these kinds of viruses are in a lot of crop plants, we think they probably confer some benefit, like perhaps that longevity, that made farmers breed and plant them during early agriculture.

What are some other ways viruses benefit their hosts?

Many plant viruses confer drought tolerance or cold tolerance to plants. We don’t always know how this works but, for example, elevated sugar is very common in virus-infected plants. More sugar would allow the plant cells to retain more water, protecting them from drought. And you know, things that are really sweet freeze slowly, so extra sugar would make plants cold-resistant.

And in animals, actually in mice, herpes viruses confer resistance against bubonic plague. That’s because the herpes virus, dormant in the mouse, turns up the mouse’s immune system and makes it better able to fight the plague.

Similarly, in people, hepatitis G virus may offer some protection against AIDS. Hepatitis G, now called pegivirus or GB virus C, is quite common in humans, and isn’t known to cause any disease. But it does affect the immune system in a variety of ways. If people are infected with hepatitis G first, and then HIV, it takes longer for it the HIV to progress to AIDS.